Thanks for all the feedback, I was still not seeing the MMC interface. On the chance that I broke something I started over.
I deleted my Win10XPE folder
Re-downloaded Win10XPE_2020-11-20.7z, and extracted.
Pointed to my install media
Pressed the big blue button.... no other changes
The MMC interface is not showing in the Apps section of the build tool, The option to download Optional Apps is selected. The item for diskmgmt.msc is in the pecmd.ini...
After launching the ISO, the disk management applet is installed and part of the start menu
Furthermore, what I am hoping to do is have the Applet on the desktop, not the full MMC Interface. I was able to drag the applet out of the Start menu to the desktop for an example.
I must be missing something basic
Screen shots below.