I think we're assuming that they used GPT3 (or whatever) on code that was stored on github.com in accordance with their TOS, which means (if I understand correctly as a non-lawyer) that they didn't share it under the terms of the GPL, since they can't, but if whatever licenses the code already had attached were not sufficient, that code was also licensed to github for the purposes of indexing, analysis, and sharing with other github service users.

Does this mean that if you picked up GPL code from gnu.org and put it in your public repo, you're in violation of the GPL for illegitimately licensing that code to gihub under a non-GPL-compatible license? Maybe!