I might be mistaken, but I remember reading Apple stating "the mMP is not a product for this *fiscal* year". Companies close their FY generally around March/April, therefore I would not expect them to tell us anything before.
Apple was talking calendar year.
"...
. This time around, Boger was succinct: the promised Mac Pro will be a 2019 product.
“We want to be transparent and communicate openly with our pro community, so we want them to know that the Mac Pro is a 2019 product. It’s not something for this year.” In addition to transparency for pro customers, there’s also a larger fiscal reason behind it.
“We know that there’s a lot of customers today that are making purchase decisions on the iMac Pro and whether or not they should wait for the Mac Pro,” says Boger.
[/quote]
Nothing there is about Apple's or some other specific company's fiscal year. If not marked out that way then the default of calendar year is most appropriate.
Hence, their engineers might have developed a prototype with a Xeon W from an iMP, but updating the board and the chipset to the new CPU/GPU available at launch shouldn't take much longer than what a typical PC OEM would need to come up with a new product. They had (at least) three years to develop it. I get they want to do it Apple-style, therefore needing time to develop a concept and its USPs, but if they need more than a few months to update the mMP to a newgen CPU/GPU, then the project is already a failure, IMHO.
The three years span of time is immaterial if there aren't folks assigned to the project actively working on it. Apple has a functional skill matrix organization. If folks are pulled off of project X to fight the fire on project Y or assigned more projects than they have bodies to distribute/allocate to them, then things will simply take more time.
It probably isn't solely external component partners timeline issues.
Apple has had "time enough" though. But PC OEM often have longer than the 3-4 month horizons many folks have expectations on these boards. The bigger difference is keeping the development pipeline fully resources and allocated sufficiently far in advance and steady.