When bots have human names and human profile pictures they are most certainly pretending to be human. But even if they do not have these, by not explicitly mentioning they are bots on social networking sites (including sites like reddit) they are effectively pretending to be humans as well, because like you mentioned people assume that other users on the site are human.

It's a false assumption. People shouldn't assume anything, there are plenty of places to do legitimate research, political or otherwise, social media isn't one of them

Technically, the humans running the accounts are pretending to be a lot of different people. But since the HN audience is aware nobody has achieved artificial consciousness, we casually attribute characteristics to the puppets, not the puppeteers.

"social media" has "social" in its name, which implies the presence of people. Hanging out with a bunch of computers is not social.