Anthropic seems to be challenging the traditional way math gets published. As far as I understand, these results did not get submitted to journals, and did not get Arxiv preprints; they are released only as self-hosted pdfs, and we don't even know the names of their authors.
The canonical reference for the counterexample to the Jacobian conjecture is a tweet with no puntuations nor capitals.
I've cited letters from Serre to Tate in my dissertation: this is the source for what's known as Faltings-Serre.
As far as I can tell, Arxiv does not allow an AI to be listed as the author, so publishing there would not have been an option.
https://blog.arxiv.org/2023/01/31/arxiv-announces-new-policy...