That’s good its only usernames and post content. As long as it is only publicly-visible content, then really it makes no difference if it was a search engine, an AI, or a human, that saw some content and spread it or derived something from it.
I would be concerned about private posts/categories that are for certain logged-in users. If sensitive business discussion is happening, and that stuff goes to an AI, well now the AI can present those ideas to anyone else who may ask for business ideas. Or similar.
My site is for an open source project, so the more data gets sent out for AIs to learn from, the better for helping everyone out.