Err, I've never seen Username claim omniscience or argue that his explanations for phenomena are anything more than possible-- and more than that: that his opinions are anything more than provisional and subject to change if they fail to line up with observations. His arguments are very much based on hypothetical reasoning, and are not dogmatic and rhetorical like Tom's. I'm sure if you look hard enough you'll find him explain his reasoning for why sustained spaceflight is impossible.
It seems to me his motivation is discovering whether the earth actually could be flat, where Tom's motivation is convincing other people that it is flat, or just plain trolling.