There are "Pro" Macs and Macs that say "Pro" on the front. Not many people are going to spend $6k on a 27" iMac with 10 core i9, 128GB RAM, 1TB SSD, 5700 XT w/16GB etc. to update their Facebook and sort their holiday snaps, any more than people are going to buy an 8GB/256GB 13" MacBook Pro for editing Hollywood movies. The current "pro" labelling is a mess, and - as I said - a new world where the "consumer" Macs all have "M1" and the higher-end Macs all have "M1X" (or however the naming turns out) is an opportunity to sort that out.
...and there's no evidence that they'll use the same design as the 24" either. It's all speculation - the problem is that some people write their speculation as if it's the undeniable truth.
We've already seen Apple use Space Grey to distinguish higher-end hardware - the iMac Pro and the 2018 Mini (vs. the 2014 and earlier silver versions with mobile processors) and the steampunk style to distinguish the Mac Pro and XDR - so it is perfectly plausible. That doesn't mean it's going to happen.
2 Soc Options x 3 RAM options (or, possibly, 3 SoCs each with 2 RAM options, if the RAM is still mounted on the SoC) x 4 storage options = 24 different types of assembled logic board. Multiply by 7 colours - 168 versions. Wait - double that for the VESA option (likely to be more popular on higher-end machines) oh, then there's probably a nano-etched display option, double that again... 10GB Ethernet option... x2 again... and many of these are manufactured-in things so it's not really "build-to-order" and more "guess how many of each logic board and case you need to manufacture". So, yeah, there's a potential problem with a combinatorial explosion of options - with significantly lower volumes than the 24" and probably a higher proportion choosing the "BTO" options because "pro" customers have more specialised needs.
Yes there is - the aforementioned $5000 5k iMac i9/5700 XT configurations (which outperform the base iMac Pro) at one end and the Mac Pro at the other. The latter didn't exist when the iMP was released, and the 2017 iMac topped out at a 4 core i7...
Anyhow, the iMac Pro was only discontinued this year, so the anticipation of a M1 replacement may well have played into that.
The 5k iMac range covers a wide range of specs from the fairly modest base model to the $6k+ options - if the replacement doesn't cover the same range Apple will have a yawning gap between the iMac and the Mac Pro. (Of course, there's also the 'Mini Pro' rumour...)
Oh, I attach zero significance to the "mockup" which looks like generic clipart that has been around since 2010.