Cool 😎 Anything developed fro FreeBSD can be ported to Linux if needed.
Unfortunately improvements made on Linux can't be ported back.
This is due to the license terms, But this is also the reason IBM, Google and many others have to contribute back to Linux, and Apple doesn't to FreeBSD.
Apple doesn't use that much of FreeBSD, and what they use hasn't been updated in ages.
And I don't think there's much sense porting FreeBSD device drivers to Linux, I think they are different enough. And the article is about things most important for device drivers and other kernel-level things.
MacOS and iOS have Freebsd inside their kernel. The reason it doesn't appear to have been updated in ages is the problem listed by the OP: The BSD license meant that Apple could take without ever giving back. Which is what they did.
First of all the nitpicky stuff: Mac OS never used anything FreeBSD in the kernel. The kernel is XNU/mach, FreeBSD only supplies the user land. Pedantic, but we have a cliche to defend.
Anyway, I think you got the update part backwards. Apple doesn't update its side of the deal. MacOS ships with old bsd apps, simply because apple doesn't care all that much about it.
So there were parts of the kernel taken directly from FreeBSD
That would also be true for Windows NT, and many other systems to be honest, because BSD is where TCP/IP support in Unix originated, it had the best implementation (or maybe not the best, but the de-facto reference one).
and OSX was designed around it
No, that's not true, you are not paying attention.
It has its userland (that'd be Unix tools like cp, ls and find) from some fossilized version of FreeBSD and not updated a lot since that. It's not much. How do you think, would FreeBSD benefit from their fixes in ls? It's the other way around, FreeBSD's userland is much better.
Their actual kernel (XNU) sources they, despite not being obligated, release from time to time.
But say they used Linux, there still would be nothing to force them to release their drivers' sources or their GUI's sources (which are closed).
You could have made your case with Sony (I'm still not sure if that'd be of much use), but not with Apple.
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.