In short, it's an excuse so that he can justify not responding. It might be one thing if he'd just be honest about his reasons for withdrawing - that he's tired of continuous argument, or that it's not getting anywhere - but he's not. He's just catering to his own prejudices, anyone who argues with him must not be competent, otherwise they wouldn't be arguing because they'd agree with him.
It's a defense mechanism. He thinks his ideas are The Truth and everyone should believe them. It comes as a shock to him that people don't and he takes it personally. He feels insulted. So his way to deal with it is to write off his opponents in a condescending way.
He is a young man who is trapped in a phase many young men go through during puberty where they want everything to be certain, black and white, cut and dried, literal. And they look down their noses at anyone who disagrees with them.
I once knew a couple boys who liked heavy metal and believed that it was objectively
The Best music to listen to. They were very smart but socially awkward nerds, just like Oli. It was not a matter of taste in their minds. They had a whole argument based on how difficult it was to play and how complex it was to compose. And they felt contempt for anyone who preferred any other kind of music. They saw them as stupid, shallow, vapid, persons lesser than themselves. It was a desire to be Right born of insecurity and hypersensitivity. It is fairly obvious how religion plays into these feelings.
Many boys eventually outgrow this. Oli has not.