I can confirm this is still correct. When I made the booking, BA gave me one Aer Lingus booking number (always starts with "2") but when I called Aer Lingus (516-622-4222) to reserve seats they told me I had a different booking number. They did give me seat reservations, and with my new Aer Lingus booking number, I could see the seats (but not change them) online and add our missing API information (which oddly, had already been fully provided to BA and were attached to my BA FF number).