Of course we're talking of code for which that's implausible.

I think in most cases twenty lines of identical code are more than enough for that to be the case (at least if it contains a few non-single-letter identifiers).

I mean, if it uses the exact same identifiers and maybe even comments, then sure, that's pretty good evidence. However, 99.99999999999 percent of the time this will not be the case. As in, the super-super-super-majority of LLM-generated code is customized for the specific use case it's being employed in. As such, claiming any kind of infringement, let alone proving it, will be insanely difficult.