Plausible. But Apple never was a company who cares about expansion slots. Today we have a similar situation with the un-upgradable Mac Studio and the lackluster Mac Pro.The iMac Pro must have been in a fairly advanced stage of development by April 2017 while the new Mac Pro wasn't even a twinkle in their eye. My guess is that the iMac Pro was going to be the new Mac Pro, and the panicked April press conference probably happened after they shared that news with a few key developers and customers...
These supercomputers are prestige projects like super sports-cars for car makers. They are not build for practical use. Otherwise they would be simple beige boxes.