How did they target the group though, did they just scan the internet traffic and said, oh wow there is a group of people who only talk through Signal, or were there another precedence?
According to this Wikipedia page [1] that details the case they (or at least some) were put under surveillance when they returned to France after they had spent time in Syrian Kurdistan to fight against IS with Kurdish YPG [2].
How so? They were fighting against ISIS, alongside YPG, who the US government (and probably France too) supported officially. It makes sense they were surveilled at first - France wants to make sure volunteers aren't actually supporting ISIS - but they weren't and that's not why they kept the surveillance going. It was because they were an "ultra-left" group and France was apparently concerned they might attack police or sabotage phone infra or something (per Wikipedia.)
They were then arrested preemptively (after the surveillance was retroactively authorized by a judge!) and one member was held in solitary for 16 months. Despite all that, the prosecutors apparently couldn't find an actual plot to accuse them of planning, so they're pointing at their use of Tor and Signal instead. It's pathetic.
It's burying the lede because the title makes it sound like the authorities in France are going around finding tor/linux users and arresting them, when what actually happened is that the group was surveilled because they were known to have fought in syria, and after they came back they were also using tor/linux. Don't get me wrong, arresting them for that reason is still an injustice, but it's misleading to paint the whole situation as "arrested for using linux and encryption".
> the group was surveilled because they were known to have fought in syria
That's not correct. One person fought against Daech in Syria, then a few years later a dozen persons get arrested ; most of them don't know one another. You can't really talk about a "group" and certainly can't say they took arms when only one of them did (and for an arguably very good cause which was approved by the State on paper).
I agree with you the video title is misleading, though.
but ultimately that's what happened. they're charged with criminal conspiracy, but rather than alleging some specific plot, the prosecutors say that using Tor and Tails proves their clandestine nature, and that nature is incriminating in and of itself.
them fighting alongside YPG has nothing to do with why they were arrested, it's just backstory.
[1] https://fr.wikipedia.org/wiki/Affaire_du_8_d%C3%A9cembre_202...
[2] https://en.wikipedia.org/wiki/People%27s_Defense_Units