The stealing part doesn't refer to directly copying ideas (I mean what great artist would do that?), more how good artists instead are able to use other works as inspiration, "make it their own" and put their spin on it.
Great artists realize perfection when they see it and just copy it, without trying to “improve” it, usually from lesser known, less successful artists so they get away with it.
It was a reference to Apple apocryphally stealing "the mouse, windows, icons, and other technologies that had been developed at [Xerox] PARC" [1].
They didn't copy an existing product. They deprived Xerox of the market into which to launch their own product.