First, it's not an argument, it's an observation, as there's no argument the be had. Why? Because...

Second, the observation is that you can literally characterize every online discussion forum that way. It's a pointless comment that borders on tautology. Put people in a space together to share their thoughts, and some of them will end up behaving badly.

Third, it was an observation of the silliness and, frankly, hypocrisy of simultaneously complaining about the behaviour of shouting baseless opinions on discussion forums by shouting a baseless opinion on a discussion forum.

There's no admission that he's correct there at all. The existence of one bad faith actor doesn't make everyone a bad faith actor.