What is your fair use claim as a defense to a third party using their source code?

It is an affirmative defense, you to be able to argue the merits. If you publish their source code, they are allowed to come after you whether they have previously used fair use or not. It's fact specific and determined case by case.

Anthropic won half of their fair use argument in the billion dollar settlement, but lost the other half.

You can say you're just using their code to train your own models, just like they did, and they will correctly point out that how you obtained the code also matters and you will lose just like they did.