The relationship between Google and Reddit goes beyond just improving normal web search for Google - it's actually symbiotic. Because Reddit has horrible search, the best way to search Reddit content is actually using Google's indexing instead. Every time the Google algorithm feeds Reddit, it's feeding itself with more '...reddit' queries.

Further, I feel the role subreddits are playing is not too different to the role curated directories used to play in sorting and filtering the web ecosystem for Google when it’s algorithm was considered good. So while there is a lot of user generated content, i feel the real benefit of Reddit to the Google algorithms quality of output, is the human sorting of content.

So my theory follows: the Google algorithm has never been independently good, it has always been dependant on a large source of human curation. The Google algorithm only works well when Reddit is applied to it, because Google supplanted, or ignores the other places it happens on the web. Or worse, they've promoted and paid attention to unauthentic content curation through their SEO and 'trusted' sources initiatives.