Attribution is irrelevant in Authors Guild, the books were not released under open source licenses where attribution is sufficient to meeting the licensing terms. Google never sought or obtained licenses from any of the publishers, and the court ruled such a license was not needed as Google's usage of the contents of the books (scanning them to build a product) did not represent a copyright infringement.

Attribution is mentioned in this filing because such attribution would be sufficient to meet the licensing terms for some of the alleged infringements.

It's an irrelevant discussion though, the suit does not make a claim that the training of Copilot was an infringement which is where Authors Guild is a controlling precedent.