In the context of well-known oneworld lounge benefits as applied around the rest of the world, and the expectation this creates, I would say it is hard to understand and yes it does need to be explained again and better communicated by BA - as evidenced by the fact this keeps coming up (and the bad feeling it creates).

Where an airline wants to restrict access against oneworld policy as a carve-out, Qatar does this transparently on the oneworld lounges webpage by saying no frequent flyer access to its premium lounges. I don’t agree with it and it could be more prominently displayed during booking, but it’s there in black and white nonetheless.

There are other airline outstation lounges found at another airlines hub with many short haul travellers that operate without blanket bans - restrictions are seen as fluid and temporal, which CX manage effectively when needed on selected days at LHR, as with QF. Neither lounge is an order of magnitude bigger than the MIA lounge I believe.

BA would be better served by being up-front with saying their outstation lounges are for BA passengers only and owning the fallout within oneworld - rather than taking with one hand for its own passengers and not giving with the other.