This really is the big question. I can see them having only standard configs for the laptops and perhaps for a lot/most of the iMacs, but not really the iMac Pro and certainly the Mac Pro. There are high-end uses where they want well in excess of 64gb, heck, up to 1.5tb on the mac pro. It doesn't seem possible/likely to provide for that without some kind of DIMM, at some point expense of putting all on 'chip' just gets too crazy.
They probably could dedicate the on-board (on system on a chip) memory for use as a fast memory cache - but it would need some kind of cache management software/logic, and that has its own overhead. Overhead's not the end of the world of course, but at some point the overhead eats up the speed advantages.
On the other hand, it's apple and they control the operating system and SoC. I think traditionally ram has almost always been assumed to be same speed (after the established on-chip caches); perhaps apple has a plan or way to use two different types/speeds of ram without formally treating as a cache but allowing for different speeds and 'nudging' more important data onto the on-chip ram.
Perhaps they have simpler solutions in mind; I suppose you could do worse than extending the concept / use of ramdisks, with some APIs for developers that really have the need.