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The flip side (of this specific example) is that several of those projects were either started or nearly exclusively funded by Apple.

- Mach's primary architect, Avie Tevanian, was hired by NeXT to productize it, and he continued that work at Apple with XNU and the OS X strategy to replace the classic MacOS

- Apple hired Chris Lattner after he finished his PhD (where LLVM was started) and then created Clang and open-sourced it, as well as other LLVM tools like libcxx

- Apple hired Michael Sweet (creator and maintainer of CUPS) and paid him to maintain it for several years

As you say, Apple's software wouldn't be where it is today without open source. But major open source projects either wouldn't exist at all (e.g., Clang) or would have needed someone else to pay for their maintenance (e.g., CUPS) without Apple.