Earlier quoted context omitted.
They can, they can crowd out good comments with an absolutely crushing volume of crap. While humans can put out a lot of crap, bots can do orders of magnitude more. The issue is that it is an effort multiplier for when a small number of people want to target a particular forum.
How will they get those extra comments to be seen? They still have to log in and get past captchas and all the same issues that bots have always faced. Humans create a crushing volume of crap already and we already have ways to hide that from ourselves no matter the volume.
I've done some experiments with GPT-2 and it had so so performance refining with tweets. Using GPT-3 you could probably just do it using only generation.