Skylake-X is actually an enthusiast version of Skylake-W, isn't it? The Server Skylake is now, for all intents and purposes, separate from HEDT. Aside from core itself, that is.
Yes, but core can't be different. Even the dual core chips which have a separate die from the quad core ones are just cut down ones from their biggest. It's part of the reason an iGPU exist on the top 7700K like CPUs.
Here's what I mean. Totally separate dies are time consuming so they make one:
*4 CPU cores plus 72 EU "GT4", or 4+4
*No one buys the GT4 configuration so the next part sells, which is 4+3, but that isn't popular either so one after that is 4+2
*Cut the part in design and make a new die. 4+2 are basically high end mainstream parts
*2+3 has its use so they take the original design and cut the chip by 2 cores and GPU by 1, which becomes new die
*2+2 is low cost so they make a other one.
Finally if they want anything lower like 2+1 or 1+2 they take salvaged parts with deactivated cores. The die size is too small to really bother making a new die.
But server Skylake is anything but a derivative, and so is PC Skylake. Adding L2 cache which is very close to the core, or adding AVX512 is a significant layout change. Even the 1.375MB per core L3 cache is a layout change. Either SKL-X is PC Skylake with 256KB L2 and AVX256 or server Skylake with 1MB L2 and AVX512.