"This person said something I don't agree with, therefore they're an idiot." isn't nice, but it's also not ad hominem. Ad hominem is the other way around: "This person is an idiot, therefore I don't agree with them."
I still doubt that 'I consider that guy an idiot' is a reasonable thing to write in an article about smtpd filters. And _technically_ I question the example for blacklisting a whole domain, while the 'idiot' is probably just one guy (and for all I know, he might share that domain with friends or family).
If someone says something (arguably) stupid, the reaction isn't to write something utterly braindead in response. At least not among adults. Plus, 'I think BSDs are holding us back' (or even: 'BSDs suck!' if you want to condense that and distill it a bit) is targeting a family of operating systems, that example code/comment targets a person. That's just .. socially awkward at best, if we're friendly.
'I consider what this person subtly implied once as breaking an arbitrary set of rules I've constructed in my head and therefore will disregard their past and future work' -- this sounds a lot more childish to me than subtly calling Lennart an idiot due to him doing things one would consider idiocy.
And if you think someone is an idiot, there aren't ways to communicate that without calling them an idiot. If you can't call them an idiot, you can't speak of their actions as idiocy and in the end you can't communicate your thoughts, which is as good as not having them in the first place -- we enter a newspeak kind of situation.
It may be due to all the years spent on 4chan and the experience developed thence, but at some point it falls on you as a listener to actually listen to what a person is saying rather than how they're saying it, or getting your knickers out of alignment due to a tongue-in-cheek comment.