Speaking as someone who refuses good money when it involves working on Windows, because no one's offering me enough money for that level of pain...

Seriously, if you claim that a drawback of Windows is that it doesn't allow multi-user login, then it's hard not to ask "WTF are you talking about? Really, what are you talking about? Are you stuck in some circa-1995 reference frame? That doesn't even make sense!"

So I'm not sure that it was a personal remark. It may have just been honest lack of comprehension.

There is a distinction between 'ad hominem' and 'refuting an opponent's ethos'. Somebody made a factual claim about something, therefore implying some form of at least familiarity with the subject at hand. When his arguments show the he lacks even the most basic of such familiarity, then calling that out is not an ad hominem.