Ok. I said to check the Intel microcode that Apple used in the “accidentally” released firmware for the iMac Pro and here it is;
The above image shows you the header of the Intel microcode that I located in the firmware file of the iMac Pro. The data in the red box is the CPUID.
Right. You need to know how to read it, but that 52 06 05 00 is CPUID 0x050652 of the Intel Xeon processor (B-0 stepping) in the iMac Pro.
This CPUID – and the one for the H-0 stepping – can be found in the Intel® Xeon® Processor Scalable Family datasheet (see: xeon-scalable-spec-update.pdf).
Edit: I checked the processor flags in the header (97 00 00 00) and that is exactly the same as what you see on X299 platforms. You can check this with:
sysctl -xn machdep.cpu.processor_flag
With a Skylake processor with CPU signature 0x000506e3 that is 0x00000001. You can check this with:
sysctl -xn machdep.cpu.signature
But perhaps it is better to use the latest version of AppleIntelInfo.kext (v2.7) since it logs all bits from MSR_IA32_PLATFORM_ID/0x17. Or some other tool that can dump the output of MSR_IA32_PLATFORM_ID.
Pike's Universum 
Pingback: Support for Skylake X found in High Sierra DP-X – Pike's Universum
I think you meant that “but that 52 06 50 00 there is in fact the CPUID 0x50652” should be “but that 52 06 05 00 there is in fact the CPUID 0x50652”
Yes indeed. Typo fixed. Thanks!