Part of the challenge here is that "the DMCA" is a number of clauses. People who are "against the DMCA" often turn out to have an incoherent position, at least from a legal standpoint, when it turns out they just disagree with one of the contentious clauses.
This would in fact be the most contentious clause, the one about being able to take down tools that enable circumvention, the one that is historically the one that perturbs techies and HN-types the most. I think what we see here isn't so much a DMCA takedown of a single line, but a single line modification in an attempt to prevent someone trying to take down the entire ad blocker, by making it so this particular person doesn't have any standing (in the legal sense) to make claims against the ad blockers anymore.
The copyright takedown clause would be number two, but it has a mitigating factor; the DMCA copyright takedown process that you might see on a hosting site or HN itself [1] has a positive element as well, which is that by conforming to the DMCA a site like HN is able to host user content like our comments while discharging from themselves the responsibility of having to pre-filter every comment for copyrighted content. This clause has certainly been abused, and there is a justifiable case that the Feds have not been adequately aggressive about chasing them down, but on the net I still approve of this clause, personally.
(You also have to distinguish between "the DMCA" and a site's policy, which may go above and beyond. Many or most of the things that people complain about for YouTube, for instance, are their own elaborations on the theme, not the legal requirements themselves. Not all of them, though; YouTube tends to favor the big media companies very strongly when it comes to defining "fair use". But things like taking away your monetization and giving it to somebody else is a YouTube policy, not the DMCA. Or at the very least, it's a penumbric emanation of the DMCA and not the DMCA itself.)
Were I the developers or anyone with any ownership in this software, I would hesitate on putting too much stock in the idea that this was an improper use of the DMCA claim process. It was. But the reward for aggressively pushing back on that may be a proper lawsuit for violation of the anti-circumvention clauses, for which there is not a notification process but simply a legal basis for lawsuits granted, IIRC. Your reward for armchair-lawyering this DMCA takedown request could be a true lawsuit.
[1]: https://news.ycombinator.com/dmca.html - have a look at the footer of this page