That search returns a small number of links into the topic, but the topic has 58 posts so you’d expect to see 58 individual results if the /nn URLs were all being indexed. It’s possible that the spider is seeing links to posts in the topic in other posts so it indexes those individual pages?
Having said that, turning off /nn would be a nightmare for my forum. There are often long discussions about how to resolve issues which might contain multiple, this seems to work, followed a few posts later by a “oh no, it doesn’t” post. We often refer back to actual “fix” posts when someone else has that problem in future. If all you can do it point people to a page that contains the answer somewhere on it and that quite possibly contains incorrect solutions that’s not going to help anyone.
And, yes, there might be Discourse ways to highlight solutions, e.g. the Solved plugin, but my forum has 22 years of posts where only the last 12 months were made in Discourse.