Yeah, I maintain a Vulkan backend, and this immediately triggered my internal "what?" alarm.

Modern Direct3D is almost indistinguishable from Vulkan, on the other hand. So it shouldn't be difficult for them to add.

I also agree with your HLSL comment. It sounds like these guys don’t have much prior graphics or game development experience.

I'd been thinking about it, and I added another paragraph above. I get a feeling they've been targeting Vulkan 1.3 with dynamic rendering from the beginning, so porting to D3D12 would be roughly as complex as rewriting to target older Vulkan.