I'm unclear how you arrived at this being about their quality.
The statement reads simply as: apple works on many things, and doesn't want you working on things outside of apple that may compete with current r&d or future r&d.
We can disagree with them on if it seems appropriate, but it doesn't seem to be about their software quality at all.
> And yet Apple is totally fine with benefiting from the open source work of others...
And? Every other person benefits the same with the code contributed under the licenses in play. I fail to see the issue. With llvm for example they seem to be upstreaming a lot of their xcode backend stuff as they get time. So that statement of many copyleft proponents that only copyleft encourages upstreaming rings hollow to me.
Hell go talk to people from Redhat about custom gcc forks that target chips that aren't upstreamed to gcc. Just because you're using gcc and modifying it doesn't mean you'll actually be contributing the code if its all internal.
To answer you, the key part of your reply is: "...that may compete with current r&d or future r&d".
It seems to me that they are afraid (or at least unwilling) to compete on even grounds, despite likely having more budget than a single random employee competing in their free time.
Or their lawyers are extremely good at getting employment contracts drawn up. Never attribute to malice that which can be explained by paranoid lawyers that got the c levels ears.
