Apple has also made every indication they are going with their own GPU solution going forward. Not that discrete GPUs won't be an option, but I imagine those discrete GPUs will have an Apple logo on them instead of AMD.
EDIT: Also, iPhone preordered.
Not only have they basically told us they are going with their own GPU all the way through the Mac line, going with their own likely implies the market for third party cards (even if sufficiently wide PCIe slots were available at all) would be far too small for AMD and Nvidia to bother with.
I mean Nvidia ALREADY ignores the Mac Pro market, because Apple sells/supports AMD cards in the x86 Mac Pro. So there's obviously no chance they'll jump in on the new Mac Pro that comes with an Apple solution instead of an AMD solution. And if Nvidia doesn't think it is worth bothering when Apple sells Macs with something else, why would AMD not reach the same conclusion? Both have their hands full selling all the cards they can make to miners and the HPC crowd. Introduce a Mac card and the next day someone will be hacking it to figure out how to plug it into a cheap PC and mine crypto.
I have said before don't be too terribly shocked if the ARM Mac Pro does not include PCIe slots at all. I think they will include some 4x slots but I feel that's far from assured. If the GPU is integrated, the only other things you need for PCIe are add in network and storage, and unless 100GbE is a must-have for segments of the Mac Pro market (I have no idea about that, and if so they might offer an optional 100GbE fiber port like how the Mac Mini has an optional 10GbaseT port) any network < 100 Gbps or fibre channel external arrays can be connected via Thunderbolt.