That’s not what this commenter was doing, though.

So what do you think there were doing, exactly?

Let's break their comment down, and then you can point out the part that is "extremist".

14,000 sounds like a big number, until you realise that there’s many millions of routers.

This is 100% accurate, especially in the age of Mirai-like IoT botnets. 14k is pretty small nowadays. Variants of Mirai (e.g. Midori and Aisuru) had 300,000+ devices.

Asus is not known for backbone routing

Correct, this is a pretty low-danger botnet due to being low-power consumer devices, even if it's difficult to clean.

so while this might be happening, you have to ask yourself, is this the biggest threat across the internet,

Less fair, because it is still news, and Ars is a tech news site.

or is this article intended to serve another interest?

The part I assume you take issue with, but it's also a completely fair question (and is in fact precisely "telling people to question the purpose and bias of news"). The article made the deliberate choice to name-drop BitTorrent and IPFS, despite them not being related other than them also using DHTs. I understand the writer may not have been intending to draw a "malware <-> bittorrent" association in the readers' minds... or they may have. It's sort of like saying, "the killer drove an Audi, much like Nico Hulkenberg". That's why you have to critically question news.

what’s the point of this? To me it seems like an argument over the semantics of a word which I honestly couldn’t care less about

The point is that you immediately jumped to calling them an "extremist" for what seems a pretty innocuous (if not particularly useful) comment. We generally assume good-faith around here, and calling people "extremist" for questioning an Ars article doesn't seem like that to me.