You’re not wrong that they aren’t directly correlated. I can see the argument though that making Notes look exactly like a yellow legal pad was not an automatic gain for usability compared to say, wordpad.exe with its super simple toolbar that showed you every option.

I think a fair take is that going full skeuomorphic may take some options off the table for you because they would conflict too much with the metaphor you’re leaning hard into.

I’m still 100% Team Forstall though, considering the massive violence that ios7 and its copycats have wrought on usability.

Skeumorphism where "things look like real things" can be fine. Complete skeumorphism where "things act like real things" is user hostile.

The poster boy for that is the old QuickTime Player, which had a volume wheel and a drawer for shortcuts. https://www.johnforde.com/teaching/shame/qtime.htm

Some skeuomorphism is user-friendly, like buttons that animate like they're clicked when pressed down. Other skeuomorphism was user-hostile, like the various mouse-only interfaces where everything had to be dragged around to work.

Extremes are usually wrong. Skeumorphism and flat design are both user hostile; Windows 95 is a very good compromise between the two.