You may want to re-read that a bit more closely. Apple stole it from XEROX, but XEROX didn’t copyright their work and take legal action in time to enforce it.
So Apple STOLE their design from XEROX.
I am speaking about Apple v MS not Xerox v Apple. In the case of Apple v MS it was clear that Apple could not patent the idea of a GUI. But could patent aspects of it(such as the findings against HP).
Doesn’t sound like they ever got to the issue of if Apple stole from Xerox. Since Xerox received compensation for Apple looking at their project it isn’t a foregone conclusion that if they had sued in time they would have won.