I wish he had mentioned Haruhi though (not the Haruhi subculture). That's the most extreme case of a single mind generating the entire world in which they exist, and having to keep itself distracted from that fact so it doesn't all come crashing down in meaninglessness.

My own opinion of _Haruhi_ is that it is (or at least, should be) a more positive retelling of Mark Twain's "The Mysterious Stranger", and Haruhi is an ordinary girl made special by her wish to be extraordinary: https://www.gwern.net/The-Melancholy-of-Kyon

I haven't read the LNs, and have no opinion on whether it's Haruhi's mind or Kyon's.

Either way, after watching it, I was left with a feeling of emptiness and the sense that life is meaningless if you're not able to interact with other minds that are independent of your own but can still understand you.

If the world is wholly a product of your own mind, and there is no objective reality, you had better delude yourself in to believing that the other minds that populate it are independent. Otherwise you'll constantly be destroying and recreating the would in search of novelty because you've given up on meaning.

The best way to convince yourself the world is real is if the other beings in it know things you don't know and can do things you can't do.

Could someone explain what this means and what the difference is between "Haruhi" and "the Haruhi subculture"?

The internet returns https://en.wikipedia.org/wiki/Haruhi_Suzumiya but that doesn't help.

"Haruhi" refers both to the show, which asks some existential questions in line with the article (there's one line quoted), and to the show's main character. "The subculture" in refers to the fandom, which is... you know how crazy fandoms can get, and this one got there.