This is a common narrative, but it depends on an empirical question of whether hate spreads online and whether this motivates action. Fortunately, studies suggest that online talk does not motivate action:
To summarize: the internet does not accelerate the process of radicalization, it does not provide opportunities to self-radicalize, and it does not allow radicalization without physical contact with other radicals. So the empirical evidence does not entirely agree with your characterization of 8chan's role in radicalization.
But that isn't the point you're replying to; the point you're replying to is about "these boards" (ie. these venues), and makes no mention of their internet-ness being a factor.
The report, crucially, does therefore not seem to contradict the post you're replying to, as that post is about radicalization in mixed-topic venues in general; this one just happens to be on the internet.
Furthermore, it seems pretty clear that the OP was specifically referring to online boards, and this forms the context of pretty much this entire thread and every discussion of this topic here and elsewhere about the spread of hate online.
> 8chan might be compartmentalized in a way that allows it to become an echo chamber of hate, but it also is highly connect to boards about general topics like video games, TV, and movies.
Nowhere in the OP is "the internet" specifically mentioned as a factor. It's all about the general concept of mixing it into other topics to make it palatable (which is precisely how radicalization usually works online and offline, see also eg. biker gangs), and an online message board just happens to be the context in this particular case.
And like you said, there is nothing internet specific about this distinction. The same thing applies if white nationalists are recruiting in the physical world. There is a lot more potential for recruiting new members at the local bar than their is at a KKK rally. I think some of us just want the bar owner to stop allowing those white supremacists to use the bar as a recruiting ground because they are turning violent.