Both paragraphs: your confusion might be in the misinterpretation of how seriously he took posting here. Your hindsight sentence is the key: some might post appearing they are serious and conscientious about it, but really only have enough commitment to post something rather than a consistent effort to be thoughtful, opinionated, and constructive (in some sense). Obviously, the ability to post with a low commitment but still be able to be these things is higher in some than others and Meletus had more than most (if I'm on the right track), but it might account for the paradox you see in his posting history.

Fritz: I read a post from Fritz once that had an opinion about my personality/posting content that I had never know of myself and I knew to be true as soon as I read it. This was long after I had stopped posting regularly and the comment was almost an afterthought by Fritz. But it was maybe this ability to read people and show them something of themselves that Fritz has (good or bad) that Meletus wanted/needed as his condition progressed. I've no idea if he got it as I don't know the circumstances of their dialogue.