I'll confess, you're probably right that I'm conflating some stuff from nihilism, existentialism, and absurdism. It's been a while, and my understanding is that they were always very overlapping and informed by one another. I've just never met or even heard of a real person explaining their own beliefs in literal "We believe in nothing, Lebowski!" terms outside of memes or epithets, so it's difficult for me to jump to the conclusion that it must be what someone intends from one instance with plausible ambiguity. Accepting the premise that someone does strictly believe "nothing means anything; full stop", I don't see how that would be an action motivating belief. If "nothing means anything" is the full scope of how you relate to the world, then where's the benefit in persuading anyone else? If nihilism definitionally prohibits a "therefore" after the proposition that "nothing matters", then I don't see how it's not self-excluding. Nobody can exist in the world in a perfect state of inaction, and if "nothing matters so make your own meaning" leaves the definitionally pure confines of nihilism, then I don't see how "only I matter" or "only I and [subgroup]" matter isn't just as much a departure from that definition.

I've never called myself a nihilist because to me the "nothing matters" or "nothing has intrinsic meaning" part of the equation always seemed like an immaterial meta-issue. If you can't objectively test for whether or not something matters, or quantify the degree to which one thing matters over another, then "nothing matters" and "everything matters an infinite amount" are functionally indistinguishable to me. It's what you materially do with the motivation that I'm interested in. I don't think "bully more people on the internet" is a particularly worthwhile thing to do or encourage generally, no matter the thought process behind it. To the extent that one's "political action" is limited to online bullying, I feel like "people that talk about science", "people that talk about philosophy", and "people that don't believe a god" are pretty poor proxy groups for the people in real life that actually have the political power to make the world worse, unless you're identifying "intellectuals" rather than "capitalists" as the final boss of class struggle. It just feels like, if you want to make a reasonably safe materially insignificant net positive contribution to the class struggle without working too hard or thinking too much, you'd be better off shoplifting a pack of gum from a business, or throwing a rock at the most expensive house in your neighborhood or something.