Your argument - "here's the internal layout of the iMac Pro, now show me where the RAM door would go" - is fallacious. If "must have user-upgradeable RAM" had been high on the designers' list of essential features then it could have been taken into consideration from the start and would have influenced the layout of the logic board (as it obviously did during the design of the regular 27" iMac). If you're claiming that they wanted to have a RAM door but discovered some unavoidable technical snag during the design then, as they say, "citation needed". The fact that you can't easily kludge a RAM door onto the final design proves nothing.
Meanwhile, the fact that the RAM is socketed doesn't mean Apple wanted to allow aftermarket upgrades. Are the appropriate ECC RAM chips available in a suitable form to solder directly to the logic board - and is that cheaper than buying pre-made DIMMS? I don't know. What it could also mean is that rather than try and anticipate how many 32GB, 64GB, 128GB models (of each CPU/GPU variant) they will sell at the logic-board-manufacturing stage, they could configure the RAM at the final assembly stage - maybe on-demand.