I also wondered why the author set it up to echo the existing comments, instead of for example building a bot that asks people for evidence supporting their claims, or tells people they appreciate them, or whatever else one could possibly think of for a bot to do.

I think you're right about it pushing them further over the edge. The other thing is, what's stopping them from creating more and more image boards/chans in the longer term? Will this end up being a whackamole game between the botters and 4chan posters?

It also begs the question. What if they were botting the communities you personally enjoy and love? Is it ethical for them to do it?

You could argue that bots are expected by the hosters and people there, and they are happy to be experimented with (ok, not sure what's the evidence for that argument), that's one thing, but another is releasing the model freely and not being bothered by ethical concerns.