Very soon, the domain of bullshit will extend to actual text. We'll be able to buy HN comments by the thousand -- expertly wordsmithed, lucid AI comments -- and you can get them to say "this GitHub repo is the best", or "this startup is the real deal". Won't that be fun?
You're forgetting the millions of additional comments that will be written by humans to trick the AI into promoting their content.
Even worse, currently if you ask Chat GPT to write you some code, it will make up an API endpoint that doesn't exist and then make up a URL that doesn't exist where you can register for an API key. People are already registering these domains, and parking fake sites on them to scam people. ChatGPT is creating a huge market for creating fake companies to match the fake information it's generating.
The biggest risk may not be people using AI-generated comments to promote their own repos, but rather registering new repos to match the fake ones that the AI is already promoting.
E.g., if I create a great paintbrush which creates amazing spatter designs on the wall when it is used just so, then, beyond a point, I have no way to control the spatter designs - I can only influence the designs to some extent.