The pushed-out timeline may have something to do with monitors and T2 chip production.
It doesn't seem likely Apple will sell a Mac Pro unless they can also sell you an Apple branded monitor at the same time. I know there's the LG ones in the Apple Store, but they don't maximize profit potential that way, as opposed to one with a premium "look" to the enclosure that matches the Apple space gray aesthetic of the iMac Pro. They are selling space gray accessories now. FreeSync 2 might also be on the table now that select Radeon based eGPUs are officially supported.
T2 chip production may also be a factor. Apple may only have supply for the iMac Pro at this time because the Mac Pro was not a consideration when they engineered the iMac Pro.
As for the conspicuous use of the word "modular" the implication is that the Mac Pro will be made of modules. Yeah, I know, not a great leap of logic there, but what I mean is separate enclosures as opposed to all inside one case. Perhaps the idea is that the CPU module offers a choice of a LGA2066 module or a LGA1151 module, which looks similar to a Mac Mini, slightly larger to accommodate the appropriate cooling solution, which then sits on top of the eGPU module with the choice of enthusiast cards from RX 560 up to Vega 64, or workstation module housing one or more WX 9100 for example.
The fact that there are no info leaks from sources connected the manufacturing side may suggest that development has yet to get that far. Its fun to speculate, which is all this is, but if you read all this I'd love to hear your predictions.