When you consider the price of the flagship iPhone (whenever it launches) it is a bargainMaybe, but the iMac Pro is a €/$ 5000 computer. Not that big a market for those...
I agree though the market will not be massive, but quite a few on older iMacs have been waiting some time for a decent upgrade, and compounding this is Apple are pretty stringent when it comes to suppliers so AMD probably has to play it safe from a logistics-channel perspective.
So if AMD is not at full capacity in terms of production for Vega I can see custom models being hurt and at the end of the queue.
Also are the Radeon Instinct available from certain channels or HPC/datacenter service providors yet, I can see this also having priority over custom Vega models if there is a logistics component shortage/yield challenge somewhere.