If I was buying the new Mac Pro I would get it with the 76 core GPU and 192GB unified RAM which makes it $9,599. To get the DIY workstation up to par with a 36 core Xeon W9-3475X, 128 GB DDR5 ECC RAM, and a Radeon Pro W7900 with 48 GB VRAM pushes it to about $10,705. The PC workstation would be faster, but costs more and can't run macOS. The Mac Pro besides being non-upgradeable is a better buy.
The 13900k is faster than M2 Ultra. And it's much cheaper.
Is the 13900k a workstation chip? No. But neither is the M2 Ultra. The M2 Ultra doesn't have the PCIe lanes a workstation chip would have. It doesn't have the RAM capacity a workstation chip would have. It doesn't have the ECC a workstation chip would have.
There is no reason to price the Ultra against a workstation chip because M2 Ultra isn't in the workstation class.