Yep, it really does look like if you want ray tracing, you are better off sticking with Nvidia.
For NVIDIA partner titles, absolutely.
And of course there's that promised Ryzen 5000 boost where the CPU can access the entire 16GB of GDDR5 memory in the graphics card as one unit.
This is a mostly artificial limitation. Any system with appropriate firmware support for the optional PCI-E spec resizable BAR, combined with GPU drivers that take advantage of it, should allow "Smart Access Memory", or a non-AMD equivalent (which
NVIDIA announced almost immediately after the RDNA2 preview), to function. A combination of marketing and compatibility concerns is likely why it's limited to the Ryzen 5000 series CPUs and 500 series chipsets, currently...probably the same reasons that ray tracing only currently works on AMD hardware in Godfall and will initially only work on NVIDIA in Cyberpunk 2077.
Cool. I would assume that DirectX will fully support whatever AMD or Nvidia do, but you know what they say about assumptions... and I don't have a card to test.
Both AMD and NVIDIA contributed to the DXR spec and both are making fully compliant parts. Any incompatibilities are down to drivers and and game developers. Any truly DXR compliant title is supposed to work on any DXR compliant part, but as you note, things may not always work that way in practice.
Deliberate/artificial exclusivity is a real risk, as we are already getting a taste of. Hopefully, this doesn't persist long after the launch of titles, but I can see poorly supported games never having RT enabled on competitor products, even if there is no technical reason why they couldn't.