Where I had written that "Everyone using a computer with a mouse and icons today -- whether Windows, Mac, Linux, or something else -- owes [the Mac OS] a debt, for showing the way," Martin countered that "Hey, let's not forget Xerox PARC, where the graphical user interface actually originated."
While Xerox developed the first GUI, and computing pioneer Douglas Englebart invented the mouse long before that, Apple was the first to bring icons, windows, and the mouse together in a product that people actually bought in significant numbers. (Apple itself had brought them together in the Lisa too, in 1982-83, but it made little impact on the lives of regular computer users.)
Bruce Horn worked at Xerox's Palo Alto Research Center (PARC) from 1973 to 1981. After that he worked at Apple. Here's what he had to say on the subject. There's another perspective from Jef Raskin, who initiated the Macintosh project.
The point is, there's a substantial difference between inventing something and making it viable. Only rich tinkerers could afford an automobile before Henry Ford developed an assembly line to make cars cheap and reliable enough for everyone. The Model T was the revolution, not the the work of Markus, de Rochas, and Benz.
As this Stanford University page says, underlying (true) claims that Xerox invented the graphical user interface "is the belief that commercialization is technically trivial: whatever work the Macintosh team did, it couldn't have been as important as the basic research done in Douglas Engelbart's Augmentation Research Center and PARC's Computer Science Laboratory."
Apple did not invent the mouse. Nor did it invent windows and icons. But Apple made the Mac, made it so people could buy it, so programmers could create programs for it, and so it didn't break. Those are the reason we all use mice, windows, and icons today.