That mockup was probably a literal "mock" where it still keeps the same thermo envelop issue, which Apple just admitted they **** the bed on. There is absolutely no chance that the new design will be even close to that, at least not in a compact form factor.
I am still pondering how literal should we take from the word "modular". If Apple is committed to the ecosphere approach, with their investment into Thunderbolt 3 on the MBP (and probably down the road on iMacs etc), there is some benefits in building an externally "modular" Mac Pro where components are inside enclosures where they stack and connect via some interface externally, including TB3 when it serves. This way, for example, a display can have its dedicated GPU, and whatever TB3 equipped Mac can use it, including an MBP or even an MB. Multiple GPUs, multiple drives, or specialty interfaces like card readers can be part of a stack. This also means liberating these "components" down into the consumer Mac user market where the "single digit Mac Pro" market segment was probably too insignificant to focus this much energy on.