Sort of true. But when you first issue the ticket then Emeco or BA CAI will want to have control of a PNR containing at least your LHR-CAI flight and (in the case of Emeco) at least one trans-oceanic AA flight.

There is, of course, nothing to stop you subsequently changing the AA flight to BA.

[Just a BTW: in the last 8 years I have done about 14 ONEs in D or A. I think 4 from Emeco, 2 from BA CAI, 1 from CX CDG, 1 from BA BRU, 4 from CX in HKG (of which one, at the cost of great psychological trauma, consisted simply of HKG-JFK-LHR-HKG), 3 from CX in TPE.)