To be clear, they're talking about ideas and the adaptation of those ideas, not literally stealing cash or property.
“Good artists borrow [by modifying ideas], great artists steal [by copying ideas].”
“Good artists borrow [by modifying ideas], great artists steal [by copying ideas].”
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.