iMacPro does use Xeon chips that are similar to i9-7900x (Skylake-X) chips, but Apple doesn't offer a 12-core iMacPro. Below is a chart comparing 1st two generations of Skylake-X chips.
I own a 4,1 MacPro which though the years I upgraded to a 5,1 MacPro through various CPU, RAM, GPU and firmware updates. I love that MacPro. Two years ago when the iMacPro was announced/released I seriously thought about buying one, but the cost was too high for a machine I could never really upgrade.
The new MacPro is going to be an awesome machine. I know this is the case because last December I made a machine that runs MacOS with off the shelf parts that are very similar to what is currently in an iMacPro, without any thermal throttling issues and with tons of upgradability.
I know I am happy with what I currently have. I record my son's basketball games so we can watch them again later. With my old MacPro it took hours for iMovie to make a video of the complete game. With my new machine, the process now takes minutes, instead of hours. I can only imagine the time the new MacPro is going to save with its newer processor and improved GPU.