I mean, Terry was nuts and has his moments but you can't deny the dedication he had in making TempleOS and the ammount of work on it is nothing short of incredible. And even TempleOS has some little groundbreaking gems that other OSes can't even dream to have - not even Plan 9.
All this to say that I don't think developing a custom LLM can ever be compared to developing a complete OS. But I don't know, don't take my word on this, I'm just a graphic designer