It is, but it is also how the Manhattan project became "visible" to intelligence community - scientists related to nuclear research stopped publishing papers.

it's not proof of existence, but it's suspicious unless there is a good reason why cryptography should be more immune to AI driven progress than other disciplines. If i knew an elephant had walked through my porcelain store, and miraculously all the blue cups survived intact while every other type of cup was smashed to pieces, I'd guess that someone took steps to preserve them.