I’m failing to understand what your statement have to do with the early-2008 Mac Pro support of AMD GPUs and the Apple AMD driver requirements.
I'm thinking they are referring to the fact that Windows users can use new AMD cards, without SSE 4.2. Phenom II's I think are limited to sse4 or 4.1.
You can use an RX460 in a 3,1 if you only use Windows on it. Why have a mac pro at that point?Meanwhile we have a Phenom II PC with RX460.
The SSE 4.2 thing is only forced to DX12 use in Windows. The card still works for basic use and anything not inDX12 mode. However if I was using a pre sse4.2 PC I'd just use a Geforce to avoid the issue all together.