The occasional piece of software might be a trade secret, but a person downloading a preexisting leak isn't affected by those laws.
So now consider two questions:
1. You actually didn't use an LLM, but they believe & claim you did. Who has the burden of proof to show that you actually own the copyright, and how do they do so?
2. They write new code that you feel is based on yours. They claim they washed it through an LLM, but you don't believe so. Who has the burden of proof here and how do they do so?
My take on the answers (I am not a lawyer):
1. You copy their code. They bring a copyright claim (let's assume this isn't a DMCA thing and they're actually bringing a claim to court). Your defence is "the LLM wrote it so no copyright attaches". Since they're asserting their copyright claim, they would have to provide evidence for that claim (same as in any other copyright case), including providing evidence that a human wrote it (which is new, and required to defeat your defence).
2. They copy your code. You bring a copyright case. Their defence is "I used an LLM to wash the code without copying". Since they're not disputing your copyright claim to the original code, you don't have to defend or prove your copyright. But you do have to prove that their code infringes on your copyright, which would mean proving that the LLM copied your code when creating the new code. This has been done before by demonstrating similarity.
