Why do people keep repeating this? First-to-file versus first-to-invent has nothing to do with the patentability of an invention. It concerns who gets the patent if two substantially-similar but otherwise patentable inventions are submitted for patent at the same time.

According to my understanding, the current US patent system is often summarized as “first inventor to file” meaning the named inventor must believe herself to be the original inventor.

Prior art is still relevant WRT patent prosecution. The AIA made several changes to patent law, perhaps most notably making the effective filing date the determining factor for purposes of applying prior art, instead of the invention date. This means interferences no longer occur. There are also now different post-grant procedures for challenging patent validity.

Source: am a patent agent Disclaimer: this is not legal advice