The reason is because T4 staying closed is putting pressure on the remaining terminals at certain times of day during the planned Summer schedule. The majority of ex-T4 flights are going into T3.

T5B/C gates actually have relatively low utilisation in terms of number of flights using them per day (even pre-Covid), so moving a few AA widebodies over to make room at T3 is just Heathrow trying to maximise use of resources without having to reopen T4. Nothing to do with premium passenger experience, unfortunately.