Why do COMB UUIDs not have this problem? Do they not contain a timestamp?

It just orders ids based on a monotonic clock that wraps after a certain time -- it's enough to get a lot of of the performance benefit of ordered keys without directly exposing the unix wall clock time they were created (absent a dedicated attacker with the ability to create ids without you picking up on it for a timing attack, I suppose).

Whether that tradeoff makes sense probably depends.