The real question is whether it constitutes derived work, though. And that is not a question of similarity so much so as provenance - if you start with a codebase that is GPL originally, and it gets gradually modified to the point where it doesn't really look anything like the original, it's still a derived work, and is still subject to the license.

Similarity can be used to prove derivation, but it's not the only way to do so. In this case, all the code that went into the model is (presumably) known, so you don't really need any sort of analysis to prove or disprove it. It is, rather, a legal question - whether the definition on the books applies here, or not.