Lots of those people had an agenda, convincing the government they live under to spend more on armaments. Lies were a means to an end. People got a lot more specific about public debates on military spending back then then they do today, mainly I think because the weapons were simpler. Anyone can count inches of armor, tons of warship ect... while today the vital details are much less obvious and more complicated. So debate tends to center more on raw costs, and not specifics of any one system or even what the 'costs' actually mean or pay for.
The 17in thing is an open question, it may have been disinformation, it may have just been the result of people rounding up 42cm at some point or totally invented. The British were pretty successful about plain disinformation at times too, they had the world convinced a 20 inch gun battlecruiser was coming for about two years in the war. Reports also circulated that the next generation of German battleship would have 17 inch guns around 1915; and more implausibly it was said that existing ships would be upgraded to take these guns by riveting on a complete second hull skin to make the ships fatter and able to support the additional weight
. I have a drawing somewhere from a US paper showing how that was supposed to work… which it would… for the price of a whole new ship. That bit you can throw under the dumb writer category.
But anyway in the days before aerial reconnaissance was so prolific though, as well as decent portable cameras, it’s really not that hard to conceal static artillery pieces if you had the will. They are almost always deeply indoors at the factory, the proving grounds are isolated, the coastal forts are isolated, any time you transport them you simply wrap the barrel in a large amount of cloth or other material so nobody can be sure what it is. The 42cm Gamma howitzers and Paris gun show what can be done, neither was known to the allies until it fired its first shot in combat. Many German of the modern coastal batteries were located on offshore islands and completely surrounded by sand dunes in such a manner that even a boat cruising right offshore wouldn’t be able to see them in positon.
17 inch really wasn’t that big a stretch so people believed it, Krupp had been making successive models of 40cm guns for fortresses and warships since the 1860s (1867 worlds fair was the first one I think) even though Germany itself never bought any, why doubt reports that he’s gone 2.5cm bigger and finally won over his own government? It’s actually totally possible that Krupp DID build such a weapon too… unlikely but possible. Its also worth considering that, given that L20a was to be laid down in late 1918, Krupp must have either begun work on a prototype gun by that point, or actually finished one, if the ship was really to be built. Making a new naval gun took just as long as building a battleship, longer sometimes. So did such a gun ever exist, perhaps even well before 1918? Nobody seems to known for certain. I dunno how many Krupp archives exist today, but it is known they destroyed many of them at the end of WW1, and others were lost in WW2 air raids. We also know the Krupp staff buried several guns on the proving grounds at the end of WW1 and got away with it. This is how one of the original fixed, rather then wheeled mount 42cm siege howitzers survived the war and was able to be used in combat in WW2. It never saw action in WW1!
Gunmakers often made prototypes about which little or nothing is now known, and the job of destroying all information and committing all key personal to secrecy on the Paris gun is amazing. Hardly more then a dozen photographs have survived of the thing, and no drawings or technical details of a serious nature. All the known and supposed designers died without leaving notes even well after the war ended. Japan had that 48cm gun that never left the proving ground in the 1920s for example, and Nazi Germany had Krupp build a 53cm gun which never left the proving ground either. Nothing was known about these weapons until the respective nations were overrun in WW2. So secret guns are totally possible. Deploying more then a few in secret is less likely.
As a side note, a few questions about ships that have bugged me. The Bismarck design is said to be an "upgraded Baden" but it seems to me as if she is more based on the Mackensen and Ersatz Yorck class instead of the Baden.
She is indeed closer to Ersatz Yorck in specifications then any other ship, though with far more deck armor. Certainly Bismarck is more like battlecruiser and less a battleship. Her vertical armor is very much like WW1 battlecruiser in thickness, and rather painfully thin on some surfaces like the barbettes but she's not really directly based off any ship. She is merely was a fairly direct continuation of WW1 design practices shown in all of those ships. I think Baden is mentioned so much simply because she and her specifications are better known and documented in books like Janes then the incomplete battlecruisers and its just become established that way.
Germany also had piles of studies from WW1 and the L20 series which the designs ought to have access too. The British tried to destroy all such records, but didn't get them all. BUt Germany simply did not have design teams in the 1920s for capital ships, so when new ones were formed in the 1930s they had to work off old stuff. The evolution of Scharnhorst from the pocket battleships is pretty well established, as pocket battleship plans with three turrets already existed (with no 15cm guns, and much less range, purely coastal defense ships) but Bismarck couldn't evolve out of that. The only thing the designers had to look back on was 1918 stuff.
Also, what was the potentially better design? Hood or L20alpha?
Hood is a battlecruiser, L20a is a speedy battleship matching the speeds of the first battlecruisers and the Queen Elizabeth class, but not comparable to the latest battlecruisers. L20a has much more firepower and much thicker armor, clearly a far superior ship for a stand up gunfight. But speed was very expensive in WW1 due to limited boiler technology, and if you wanted it you had to pay. If you want a battlecruiser, the revised Hood was a pretty damn good ship for WW1. Certainly far better protected then anything else in the Royal Navy. I forget how she stacks up with an a Ersatz Yorck armor wise but the latter ship would be several years newer anyway.
A huge number of studies were done for L20a, most of them highly similar, and some for a post Ersatz Yorck battlecruiser as well, some of the latter are pretty damn similar to Hood and IIRC some L20a studies were 30 knots with less armor. Germany also heavily studied clones of the Glorious and Courageous class, though with less insane lack of armor.