"And this, I believe, is why great developers tend to move towards OS X (yes, there are plenty of exceptions)"
With all due respect, this reads more like an Apple advertisement than a well-thought conclusion.
First, I find it somewhat strange to declare those who stay with FreeBSD or Debian as "exceptions", thus implying that moving to OS X is the norm. This is not backed by any numbers. There are just some anecdotes of some people switching to OS X.
Also, that statement implies it is meaningful that some great developers switch. However, that detail isn't relevant at all to understand to process. OS X is comparatively new, so of course people are still looking at it and some of them move over. And of course some of those people are great developers, as in any random group.
To make this more clear: Among the people who are switching, there are naturally a lot more mediocre developers than great developers (as in any random group), but the article wouldn't sound nearly as sensational if the author had written: "thousands of mediocre developers switched to OS X".
Also, a lot of people switch from a proprietary system (like OS X or Windows) to Ubuntu, Fedora and many other free software systems. However, those aren't nearly as celebrated as those who switch to OS X. Why is that?
This article is so strongly biased that it is hard not to mistake it for an Apple ad.