This: "...HN already downranks posts that get a lot more comments than upvotes..." is news to me. Thank you.
But I don't know if this is based on evidence.
For example, if you were on /new over the past 6 months, it was extremely common to see a "Twitter"-titled article get 20+ upvotes and never see the frontpage. People were outright obsessing over Twitter, posting everything they could find, no matter how benign or uninteresting it's content was. Locking the topic until verifiably interesting things happen was a great solution. It probably works similarly for other keywords, too.
This is old code, and the mods make tweaks here and there without warning, so the details may have changed. (I'm not sure if gravity changed from 1.8 to 1.6 (???).)
I didn't test it personally, but it feels like HN is using something very similar. And there are a few black box analysis of the sorting of the front page that got similar results. Also, minimaxir is the kind of person that is probably running an script to use the HN API to verify the claims.