it's not discoverable in the slightest, but if you hit fn-control-f2, you can access the menus via cursor keys instead of having to use the mouse

It's actually listed here:

https://support.apple.com/en-us/HT201236

And so is "open the help menu" which would take you right to the menu search widget, but that doesn't seem to work at all in Firefox or Electron apps.

Thanks for the tip, I'm aware of it, but it breaks the core benefit of Alt+X : guaranteed repeats, with fn-control-f2-F-Enter you can't be certain to open the File menu since there might be another menu starting with F

Predictability is a core feature of UI design

Anyway, personally I use a better alternative to fully recreate the Alt+X to open a menu via keyboard maestro, so I've sovled the issue for myself, but it still is a core Mac deficiency contrary to the "OSX has been very keyboard friendly" claim