Well... I am wrong in my assessment of things. I had been up too many hours yesterday after rebuilding my dead machine, and my eyes skipped past the text on Core Isolation without reading it word for word.
I went back in to check every little minute detail... setting by setting, during the rebuilding process, and sure enough... Core Isolation is turned off.
And I can't get it to turn itself back on again even to see if this breaks NIS in Defender.
The error I get is
"Memory Integrity can't be started.
There may be an incompatibility on your device."
Well yes... based on what I understand now, there is. :P If I turn it on, NIS will break (and who knows what else).
I went in to see what updates Windows Update would pick up ... and it is the 2004 update next, not the 2020-07 Optional 1909 update which is what it wanted to hand me before I logged last time.
So I guess I get to have core isolation off (yes, I checked BIOS settings and UEFI etc. it's all set how it should be).
Too bad I can't go back in time 36 hours and turn off core isolation :P