I'm mostly waiting for marking posts as read as you scroll by and hiding read posts (but only in your feed). I think this is especially important with the federated content that might come in later than when I "scrolled past" the point in time it would appear in my feed.
They probably mean that you can't just scroll through and say "okay, I have seen everything created up to the point when I loaded this page" because some posts created earlier might only appear in your feed later.
But say, I'm currently looking at my Subscriptions sorted by New. I doomscroll until I see stuff that is 10 hours old. While I do that posts could come in that are 5 hours old. But the next time I doomscroll I wouldn't go that far back because I think I've already seen stuff that old.
If what I have seen already was automatically marked as read and not showing for me, I would still see the 5 hours old stuff because that wouldn't have been marked as read.