As far as we know, the input was a file filled with zeroes

CrowdStrike have said that was not the problem:

This is not related to null bytes contained within Channel File 291 or any other Channel File.

That said, their preliminary incident review doesn’t give us much to go on as to what was wrong with the file.

You’re speculating that it was something easy to test for by a third party. It certainly could have been but I would hope it’s a more subtle bug which, as you say, can’t be exhaustively tested for. Source code analysis definitely would have surfaced this bug so either they didn’t bother looking or didn’t bother fixing it.