I'm not sure what the state of AVX-512 codegen was in the CLR/JVM back in early 2017, but given that LLVM's was still in buggy early stages, I would guess that it was a WIP at best so performance was probably not ideal (we had to work quite hard to get decent quality code generation). On KNL, you're either running AVX512 all the time or you're pretty much dead in terms of performance.
