I'm also not a native english speaker so I might well be misusing the term.
To me, it's not so much that it's harmful, but that the people it's harming, who are likely already marginalized, disadvantaged groups, are just entirely discounted and brushed off because they don't conform to some convenient but real-world-incompatible idea of how people live their identities. That harm and those people are apparently not as important as some business goal. That offends me.
Meanwhile, someone on the internet getting angry enough to use an insult like that is just a form of expression, it helps to define the emotional context of the writing, conveys some indignance, etc.
Why would an insult convey more offense than all the criticism in the article already does? I guess one might argue that abandoning a courteous "tone" of writing betrays a lack of respect. Imo the article already makes it pretty clear (especially towards the end) that respect has been lost, so the insult hardly makes that "offense" any worse. The author doesn't feel respected by Brin and feels exploited, so why would Brin be owed civility in turn?
It's also a particularly harmless insult that as far as I can tell has no vile connotations and implies nothing worse about the subject than that they're a jerk, so basically that they come off as disrespectful. It's pretty easy to convey that one thinks that someone is a jerk, or an asshole, or whatever, and it's usually considered okay to do so, so I don't see why being direct about it is more offensive rather than just, say, more abrasive or less polite.
In the situation of basically-helpless end user raging against the machine that is Google, I don't really think anonymity matters either. Unlike in a case of actual harmful harassment or abuse, I don't think Brin has a legimitate interest in discovering the whatever personal information about the insulting party, and if he thinks he does there are probably plenty of legal ways to go about it.
And anyway, whether the name on the article is the author's "real name" or not (contemplating which seems kind of ironic in this context), it's at most ~pseudonymous~, not anonymous at all. The name is definitely linked to a real identity, and anyway, authors have been getting away with completely made-up pen names for ages.
It's kind of strange to me that apparently it's more appropriate to insult someone if one puts one's real (however that is measured) identity on the line, given how wildly that varies in significance and consequences. That'd set the bar to entry at really unfairly different heights for different people.
That's a lot of words about this sidetrack, sorry for probably boring you, but I'm kinda fascinated by how people differ in deciding whether something is offensive.