They may be, but they're not using their AI knowledge to drive better features or experiences for their users. It seems like they're just using it to deliver better advertising.
The "refund" language is confusing for the purposes of advertising, and I wouldn't bother repeating it. It's by-the-gig pricing, full stop. They just charge you for data you use, at a quite competitive price.
I don't think I agree with this. Remember that downvotes on SO only have a cost when they're cast on answers, not questions, and that cost is a fraction of the gain from an upvote (1/2 for suggested edits to 1/10 answers).
It makes sense for downvotes to be free on questions and expensive on answers because questions are low effort while answers are high effort, and the community wants to encourage more answers.
On HN, everything is low effort, and everything is worth 1 point. Also, our problem with comments isn't that we don't have enough, it's that we have too many low effort/factually incorrect/abusive/spammy comments that we need to discourage. So it makes sense to keep downvotes free.
On reddit, I collapse conversations to show myself that I've read as much of that thread as I want to, not that I have any particular problem with it. I'll do it for a variety of reasons, the conversation strays into a topic I'm not interested in, or gets too technical. For me, collapsing is a positive, it means I read the thread instead of just passing it by.
While I support introducing collapsible threads, I don't think they should have any weight on the ranking.