A lot of people did.
I met Terry online in a couple OSDev forums in the early 00s, before his downward spiral. He was already quite combative, but still coherent. This was even before he named it LoseThos (he changed the name a few times after). I used and gave feedback.
Anyway: There were about a hundred people back then writing hobby operating systems. Perhaps everyone in those forums was? I was one of them. Some OSs had graphics. Some had POSIX. A few people could even run Firefox. I never got to this point. Some even had that same C64-like character of TempleOS.
Like GP, this is not to trivialize the work and dedication that went into TempleOS. But unfortunately he didn't became famous because of it at all. Most reddit people who talk about TempleOS haven't even installed the OS, and pretty much everyone that mentioned TempleOS to me in real life haven't even seen Youtube videos of it.
Which is very sad, because IMO lack of people using it is why Terry ended up becoming isolated and combative in those communities. People still care more about a cool story than a labour of love.