This type of counterargument seems to be very common in the internet.
Can you explain how valid you consider "we can't say all X" argument being in general? How can you make reasonable arguments about group or people so that this argument is not valid.
Sometimes we do want to make broad statements. "Kids like minecraft!" is kind of wrong, but it's kind of right and isn't necessarily a straw man. It'd be more accurate to say something like "About 70% of kids report playing minecraft"[1], or we can keep to anecdotal evidence: "all my nieces and nephews play minecraft". Those statements are generally correct. We still have some data problems - a kid is generally going to give you the answer they think makes you happier, and they're probably pretty relaxed about half-truths to annoying uncles/aunts asking them about gaming habits.
If our goal is to make truthful (as in, accurately representing reality) statements, then we need to be precise and avoid broad strokes when we describe a group. [2]
[1]: I'm making that stat up as an example
[2]: On the other hand, if our goal is to define an out-group and attack them, then broad strokes and vague insults are really useful and hard to counter
The parent poster didn’t say that, though. They said that the group was nihilist. A community can have a shared attitude and outlook; that doesn’t mean that outlook is shared 100% by the members of that community.
I think you have some very fundamental misunderstandings about 4chan, including who it was composed of and what both their stated and achieved goals were. As a hint, remember that 4chan is a chan made up of several sub-boards with different user niches across boards. Even if we narrow it down to /b/ or /pol/, you still have competing internal factions and different movements across time.
Nor is your quote an example of nihilism, even in context. Nihilism is a rejection of perceived realities and the certainties tied to those realities - someone who believes truth doesn't really exist is engaging in a form of nihilism. You seem to want to say something like "they're attacking social norms" as best as I can understand.