

It looks like the change happened nearly a year ago, and no one’s kicked up a fuss, so either it was done properly (i.e. past contributors were contacted and consented to the licence change, and any that didn’t had their contributions replaced), or there’s a big problem once a past contributor notices.
It doesn’t make it any more legal to fork the project without going back to the last GPL3 commit, though, as any contributions after the license change have to be assumed to be covered by the new licence, so the combined work would be under an invalid licence (as the old and new licences aren’t compatible) rather than being still covered by the old licence.
Normally, I’d completely dismiss the possibility that a licence change like this could have been done properly, but Stenzek is associated with Dolphin Emulator, which did manage to pull off a switch from GPL2 to GPL3+ by emailing lots of people and replacing a lot of code.
Most software is like this, but you also don’t get to look at the source code either.