pretty sure they didn't use the word stopgap..
p.Schiller -- " we have plans on iMac, to move that line ahead, and do great things on iMac. It’s core to our Mac business and our customers, including making configurations of iMac specifically with the pro customer in mind and acknowledging that our most popular desktop with pros is an iMac. We want to do things with the iMac in the future to help address those pro needs, and make it… not only continue, but more of a capable machine for pro customers."
c.Federighi -- "That is a pretty incredible evolution that we’ve seen over the last decade. The original iMac, you never would’ve thought as remotely touching pro uses. And now you look at today’s 5k iMac, top configs, it’s incredibly powerful, and a huge fraction of what would’ve traditionally required the Mac Pros of old and are being well addressed by iMac — whether its audio editing, video editing, graphics, arts and so forth. But there’s still even further we can take iMac as a high performance, pro system, and we think that form factor can address even more of the pro market."
if i had to make a wild guess--
i'd say they at least considered killing off mac pro entirely while having this new imac as their top spec'd system.. it's sort of obvious they've been working on this thing well before the press conference in april.. then just prior to the press conference, a decision was made to keep macPro alive and do a 3rd design.
(again - no facts in any of that last bit)
The other angle here is that apple will monitor sales of the iMac pro, and if that is a success, they can turn around and claim that there is no need for a Mac Pro. Watch this space, the future of the Mac Pro is not secure. If the iMac pro is a dismal failure from the pro community, the Mac Pro will live.