The 70 year rule is in the jam rules, it has nothing to do with how law sees public domain, it's the jam rules. The jam is clearly not just about coding, it's about the rest of the creativity related to game dev too. You can apparently use AI generators, for instance, I have used one for sound and it sounded fine. The 70 year rule is more so about you being allowed to use "classical" pieces of artwork. Anyways, I don't make these rules but I don't see an issue with them either.