I can't imagine they're using the OS sort for actual spreadsheet data. It's probably for something else like filenames or whatever, and it ends up not finding the file it's expecting in the place it's expecting.
However, relying on stability if it is not specified is a bad thing.
When possible, it would of course be best to use stdlib functionality, which would implicitly be cross-platform. Writing it yourself, or pulling in third-party libs to do it ranks way below stdlib, although I can't agree with myself whether doing so is better or worse than using OS-specific standard frameworks. Regardless, it is out of scope in this example.
Do you really think that when you sort a column in Excel, it puts the whole thing into a CFMutableArray and asks CoreFoundation to sort that?
Using CoreFoundation in such code is entirely sensible. Generalizing a sort for something like this would be a waste of time with no gains (as long as the functionality is used correctly, like not assuming a non-specified sort to be stable).
However, for the actual business logic, the hierarchy would be:
stdlib > large third-party cross-platform lib > small third party cross-platform lib ≈ homegrown implementation.