Interesting idea though I’m not sure how Google would treat having literally millions of posts to index for a site. Maybe if the posts had canonical pointing to the topic?
Seriously though, I have never had a problem with landing on a long content heavy page and using my browser’s “find” to home in on what I searched for.
But with infinite scroll that would be useless if what I wanted was in post number 100++ and I landed on post number 1.
Using the Discourse Search would be required and that might not be clear to those unfamilliar with Discourse.
As a topic loads in 20 posts at a time, maybe it could be worked out to give bots a per 20 URL to index?