Dude, you need to stop, you don't know that for sure, and you keep talking like you just came from the donut.
Look at what Amethyst has provided to us thus far -- it began with his friend trying an AMD 6K series card and it not even working to now what appears to be apple working on drivers to get that same RX-6900XT to show up in system settings on that very same prototype device -- why would apple even waste their time on that if they weren't planning on supporting 3rd party GPUs?!?!?!?!
You need to stop, and just let the leaks come through. I'm sorry, but I'm seriously tired of reading your holier-than-thou posts, and about to put you on the block list, because some of the stuff you've been saying lately has been downright annoying at this point. Not to mention, you keep starting a million threads on the same subject.
You don't have any connects at apple? That's fine, then stop speculating like you know for sure that they won't support 3rd party GPUs, let the leakers, who have the actual connects, post their leaks, and let us continue to discuss, as that is what a forum is intended for.
You don't like this? Then go find a PC forum, where you can bash on it all you want. But the folks here want to continue to support a true Mac Pro, and that is why we so passionately hope that apple continues to support us as well.