I’ve said in another comment that you got it wrong and how. It’s the other way around with things not getting updated - the stuff in MacOS is old, not the stuff in FreeBSD. But that doesn’t matter, because what Apple took from FreeBSD it actually does release among other things from time to time under their own license, only it’s of no use for anyone, because their real proprietary strength is the Cocoa layer and GUI. If they used Linux, they would still not be obligated to release the sources for those. I think you see the problem with your reasoning, knowing that.
I’ve said in another comment that you got it wrong and how. It’s the other way around with things not getting updated - the stuff in MacOS is old, not the stuff in FreeBSD. But that doesn’t matter, because what Apple took from FreeBSD it actually does release among other things from time to time under their own license, only it’s of no use for anyone, because their real proprietary strength is the Cocoa layer and GUI. If they used Linux, they would still not be obligated to release the sources for those. I think you see the problem with your reasoning, knowing that.