There is bots everywhere, it has nothing to do with the platform, it has to do with attackers having an incentive to do mass account farming, no platform is secure against it.
Super easy, just make a web-of-trust type of thing: messages are only visible to those who already vouched for you. Otherwise, you pay $0.01/per message/per user reached.
By buying accounts, you are buying reputation. By paying for the posts, you are maybe paying for reach at first, but (a) it will be costly and (b) it does not guarantee that the reached ones will spread anything further.
Yes your individual feed isn't really relevant if we talk about the masses, Reddit accounts are for sale quite cheap, HN as well, X too and so-on, it's literally just a matter of means/methodology. If I want today to do 1000 random posts talking about a certain thing, I could.
OpenAI has already been proven to be easily gamed through very unsophisticated poisoning (fake information in a web page + an edit to a wiki page pointing at it, fake information in a reddit post), so I'm not sure we shoudl hold up their efforts at data cleaning as a gold standard.