Yahoo, back in the days before Google, was a manually curated search engine. They had a team of people doing the indexing work and creating the database from which the search engine pulled results. In principle, this avoids SEO spam because the manual curators aren't going to add spam sites to the index. Reddit functions similarly because, in principle, users aren't going to upvote spam posts.

That’s not at all obvious to me. What subreddits do you visit? The ones I frequented had maybe 5% or less bot comments. I think mods generally banned the bots from those subreddits too.