"As part of doing a new Mac Pro — it is, by definition, a modular system — we will be doing a pro display as well. Now you won’t see any of those products this year; we’re in the process of that. We think it’s really important to create something great for our pro customers who want a Mac Pro modular system, and that’ll take longer than this year to do."
The Mac Pro and XDR display were machines that took a long time to build thus the iMac Pro (announced 6 months after that article) was created to fill the gap for two years. I don't think the iMac Pro was ever supposed to replace the Mac Pro, but only a biography of Schiller might clarify what really happened.