On top of that we can build even more abstractions and the net result is that this is how federation works in pretty much every "federated" system I know.
ActivityPub does some extra work to help you see replies on a thread, think "letters to the editor". This is a bit lossy sometimes though
It's true that sometimes users click over to a thread and see some replies but not all on their instance's UI. There's things that could be done to improve it, but it's sometimes mildly confusing, but not so bad, and you can click over typically to see whatever else is happening, and people learn to