The idea is that nobody is really a nihilist when they're confronted with reality. Philosophy never survives first contact with a crisis in the moment.

You can simultaneously believe nothing matters while also not enjoy the sensation of pain. That doesn't suddenly mean avoiding pain "matters". It just means you have physiological response that makes you avoid pain.

Your response, any many others here, seem to take "nothing matters" as "life is bad, so you should just die and be done with it". Which is a weird redefinition if you think about it. The real redefinition should be "life doesn't matter, so it doesn't matter if you happen to die". Which is not the same as choosing to take action to kill yourself, or even choosing to deal with pain.

That's not really the argument I was going for, but it does have merit on its own. The idea is more that, in an intuitive sense, it's very unlikely that if you examine a self-described nihilist's life and inner mental world that they'll be able to genuinely not assign importance to at least a few things. In fact, you might say a human being is physiologically/psychologically/philosophically incapable of being a true nihilist in the way they actually behave.

If we turn to the argument you outlined and that other posters mentioned in this thread, it does raise some important (no pun intended) questions for nihilists. If a nihilist avoids the pain response and assigns no value to existence or its component parts, then why not make use of a painless suicide method? I don't recall the exact details, but you can assemble a nitrogen DYI kit in barely a few Amazon packages that just puts you to sleep peacefully. In effect, it's Benatar's asymmetry argument but remixed to apply to living beings. The nihilist is not losing anything of value when they lose their life, but they can avoid all the as of yet undefined physiological and psychological pain that they know is guaranteed for their future if they continue to live.

The nihilist will take logical steps to avoid many sensations that they don't enjoy as part of a life that has no value, or there is something about that life that they value enough to prevent themselves from taking this course of action. Either way, the idea that a human being can genuinely believe that nothing matters is in trouble because suicide methods are so painless and convenient today. If we point out that human psychology and physiology prevent us from willing the act of suicide no matter how painless, we are back to the original argument as well. In the end, a human can consider nihilism while being unable to perceive it truly, just like we can consider what extra colors a bird can see without being able to truly see them.

Why not just say things matter to persons with desires. We all create our own meaning. Avoiding pain matters “to me”. Why wouldn’t it matter just because I’m not eternal?

Riffing in what you said, I rather say “you can simultaneously believe the universe is indifferent while also believing things matter to individual beings.”