I think there's a difference between a mathematical proof and legal proof. The mathematical proof would be "show that it never copies a code snippet verbatim",

I think there's a difference between a mathematical proof and legal proof. The mathematical proof would be "show that it never copies a code snippet verbatim", and you of course cannot prove that by example. Legal proof is I think different (not a lawyer). They're more pragmatic. If, observing a lot of cases where it does not verbatim copy, and, if an expert provides a reasonable argument as to why it is unlikely to verbatim copy, that is enough legal proof for a judge to conclude that the output is not identical enough to the developers copyrighted code.