When you subscribe to a Smart List, all the posts from the accounts on that list will show up at the top of your screen, as a feed. That’s the “firehose”, nothing removed. By default, Smart Lists you subscribe to also show up in your For You feed. In For You, we only show the posts that have engagement (likes, reposts, replies), so you won’t see *all* the posts from that list, just the best ones. That’s a super simple algorithm (“only show posts from this list that have at least 2 likes, replies or reposts”).
Note: So far I’m a fan.