The whole idea that they could hit anything from 20K ft was just a snow job on Congress to get a 4 engined strategic bomber approved. Congress was against it so the Army billed it as being a Coastal Defense weapon. It worked enough for them to design and build some.
Yes you are exactly right. Not only could the Army bombers not hit much from 20000 ft but their skills on ship identification were not great. There were several IJN "battleships" bombed by B17s in December 1941 when, in fact, there were no battleships there at that time. Of course the US Navy had some of those same problems also and not just with aviators.
Ok, most people think Britain was not invaded in WW2. WRONG! The Channel Islands where (and I think they still are) parts of Britain, but there closer to France and the place names are in French But the people are British.