> some degree of plausibility for a would-be cryptanalyst

But that's a very ahistorical theory, isn't it? The manuscript was written hundreds of years before cryptanalysts were invented. For its author to anticipate that would have required an impossible degree of foresight. Rather, whatever features give it cryptographically interesting properties must have been motivated by something other than a desire to confuse/intrigue cryptanalysts, with that just being a coincidental side-effect.