Given updating the logic board for an M3 Ultra would presumably be trivial, the lack of a new Mac Pro is likely telling.
The M2U MP was likely a one-off transition machine, to cover people who want to stuff a machine with internal storage and AV cards, and don't care much about GPU. Apple didn't invest that much in it, recycling everything bar the mobo from the 7,1.
Holding back the M4 Ultra for just the Mac Pro would be ridiculous; it would just hurt Studio sales rather than boost MP sales. Plus, creating both an M3 Ultra and M4 Ultra would increase development costs whilst denying them economies of scale. In any case, Apple have confirmed the M4 Max lacks the required UF connector.