Edit-Distance computational attack idea

[Anton]

Whether it can be mathematically proven or not I'm not sure, but there is certainly no such cipher known to modern cryptography.  So you're left arguing that someone in the early 15th century invented some entropy-lowering cipher that was never subsequently rediscovered (not even with the VMS to serve as an example) and indeed that appears theoretically impossible today.

Really not more impossible than the assumption that someone in the early 15th century wrote in a language in which no writings have been discovered afterwards.