I just don't see Apple or TSMC ready to launch an M3 Ultra out of the gate, though trying to do so would be an possible explanation as to why the machine is so late (as they deal with production and yield issues).
It seems that the Mac Studio is going to skip the M2 generation just as the iMac did (and I expect the mini would have if Apple had offered an M1 Pro version) so that means the M2 Ultra can only go into the Mac Pro and since Apple is making M2 Max (which implies they are also making Ultras and just splitting them into two Max), that, IMO, supports the Mac Pro launching on M2 Ultra.
Apple's professional customers (and especially current and potential Mac Pro customers) should generally understand Apple Silicon architecture by now and would understand that an M2 Ultra would outperform a non-Ultra M3 class SoC.