If you're a boltzmann brain, then anything you think you know about anything is illusory and based on nothing at all, including the idea that you could be a bolzmann brain.
I think this also applies, although less convincingly, to most theories that the world is illusory or a simulation.
In any case, it doesn't matter because you'll stop existing momentarily.
After all the Sun will swallow the earth in about 7 billion years. I like to think the things we do matter independent of that fact.
Boltzmann brains have no past and approximately no future at all.
Boltzmann brains experience meaning too. Not that is does them any good in the way that it does for biological system.
It's not like we have a planet rich in meaning and then the sun swallows the earth and all that meaning retroactively disappears. It just exists in the past, not the future, and I think that is sufficient.
I always thought it was funny when people thought that the heat-death of the universe had moral implications. Like that killing someone means they never existed, or something.
I don't think this view is uncommon, and I think people get there because they're just not thinking straight, but I don't know what they feel is at stake that they have to hold onto, that gets 'lost' if you accept that meaning doesn't persist through time forever.