I don't think I've ever seen this on any third party search tools for HN. If it's an actual feature it is highly under documented. The filtering that HN does is on all comments / stories (by post title) but you don't get to filter down by what a user has upvoted since that data is private to the user, only favorited data is public.

I don't think this is a feature creep, I think it's useful. Otherwise I'm going to have to code something to scrape through HN for me and provide that additional context, and even then I still would be missing a way to filter down on HN comment data vs my own local db of upvoted materials.