The arrogance comes from the fact that it came without any explanation whatsoever, but instead just acted as if it is the only possible position one could have. The quoted sentence didn’t have any elaboration as follow up, it was just the end of the discussion.

It’s phrased as if to say “if you gave it a moments thought, you’d see I’m right”, which is the epitome of arrogance to me.

I get this impression from Rob Pike as well… I’m sure it comes from decades of being tired of arguing with people, but he comes off as utterly dismissive, as if to say “you either agree with me or you’re an idiot”. It doesn’t help that he continually throws shade on Linux (which is approximately infinity times more successful than Plan9) as you can see from other comments in the same thread. I don’t come away with a good impression of him or any of the other plan9/9front devs. Their whole attitude seems to be “everyone in the OS/systems world is dumber than us, and even too dumb to see why they’re dumber than us. We have a perfect system beyond any reproach and you’re an idiot if you disagree.”