I don't know all the details of how this was done. But I think the big advance is that LLMs can now allow people to decompile binaries who don't know what "clean-room reverse engineering" means.

IMO either it was decompiled (a tedious but not complex task to do by hand, now automated) and illegal, or properly clean-roomed and buggy slop.

If it was me doing this, I'd anonymously dump the source onto a torrent, not parade around the media.