ammaross :
You know for a fact this 28 core CPU isn't 5.0Ghz all-core. Their 18-core can't even do that.
Intel has had another year to tune 14nm and the architecture. Part of Skylake-(E)X somewhat low clock frequencies is also likely due to Intel rushing the launches to counter Ryzen, ThreadRipper and EPYC, so the chips may have not performed quite up to par with Intel's original plans.
I agree this is unlikely to be stock. However, I wouldn't be surprised if stock clocks turned up much higher than Skylake-(E)X.