Nihilists still act as if things are true and kinda negate their own point. That guy who wrote a tome about meaninglessness and then offed himself at Harvard wrote a poem about meaning and finding it _through_ belief in meaninglessness.
That's a little besides the point, however, because I think it fundamental that it is only after experiencing meaninglessness that we are free to create meaning. How do you know what's really important to you? It's what you do after you don't think anything matters.
There are people who are _actually_ nihilists, who _actually_ cannot bind meaning functionally anymore. They are some subset of schizophrenics, I suspect.
If you really, fully believed that not meaning nor truth existed (which is a contradiction since the justification for the argument must use the concept of truth) you would be unable to function.
tl;dr the theory of meaninglessness is useful as an abstraction but it is not a practical reality.