I don't know if it was his only machine, but it's what he had during the week he was on-site. He had no complaints about the OS, just hardware problems with the iBook.
My point is that don't read too much into the fact that the guy in the articel uses a Mac. It makes sense to use a non-Windows machine when auditing Windows environments, that way you don't have to worry about your own gear getting infected on unknown networks. That just says Windows has a lot of security problems, and we already know that. All the security experts I know use combinations of Linux and FreeBSD/OpenBSD for the serious stuff.