If I understand correctly, Europeans can travel to Ireland, as it is part of the EU, without any customs confirmation. Then, because of the border between Ireland and Northern Ireland, they can pass into the UK in the same manner. So the route is more circuitous, but anyone from Europe can still fly to the US via Ireland to UK movement.
Althoug Ireland is in the EU, it’s not in Schengen, so visitors from other EU states still have to pass through passport control. The ban only applies to the open-border Schengen states (so includes the non-EU Norway and Switzerland).