My understanding is that as long as IDs are roughly the same range of the index instead of literally random, it reduces the thrashing about needed for indexing these. It probably doesn’t need to be perfectly exact. They’re talking about B-trees, so these would all be modifying the same smaller branches of the tree instead of going in all over the place.
Haha, that’s fair, someone absolutely would manage to write 1970s dates if they aren’t pulling the current time.