There was some speculation on these forums that Apple showed a prototype of the Apple silicon Mac Pro to some of their target market and got a negative response. Hence we are still waiting for an ASi Mac Pro because no one would buy one as designed. I have no idea if that speculation is correct but it would explain the delay.
They don't have any hardware yet to be used in a Mac Pro. Were these speculations based on industry rumours or some other information, or just people thinking out loud?