I agree about lumping Linux with Apple. Maybe someone can explain why they would do that.
Because it was a lazy hit piece
Easy explanation. The so-called study dealt with flavors of Unix. We all usually describe that as _nix. Apple's OSX is actually their Graphical User interface built on top of FreeBSD, which is a free distribution of UNIX, and is still around today. That it is VERY stable is without question, and this was the trait that Apple was undoubtedly looking for.
Another issue in vulnerability might be that Apple does not always (not a Mac owner, so i'm out on a limb here, hope i don't saw myself off!) use standard hardware, It's Apple, not a PC! If a piece of hardware is relatively obscure, it's probabably not going to be targeted with a given exploit, if indeed the exploit is even possible with that particular piece of hardware. Apple by and large uses proprietary hardware, contributing to the higher cost of a MacIntosh.