I was just there twice in December. The space used to be the E-concourse Admirals. The lounge is technically operated by AA, and joint-staffed by BA/IB/AA staff. Since the computer systems are different for the 3 airlines, one must check in with the staff member of the corresponding airline. Back in the Admirals Club days, the bar used to be a fully-staffed cash bar. Since food and drink are now self-serve, the bar is occasionally staffed (as you said, during peak hours) to facilitate service, but it does seem a bit out of place. Officially the staff are just there to clean up and restock (as in most BA lounges). I did find the staff there very helpful, proactively offering to refill drinks that I had originally served myself, and clearing dishes quickly.

With the demise of the Flagship, BA and IB wanted a lounge space that did not charge customers for food or drink. Entry to the lounge for AA pax is based on the Flagship Lounge access rules (int'l or TCON F, or AA EXP), I believe, or Sapphire and Emerald, or J and F pax on BA and IB flights.

It is a single space (no separate F and J space), and you are still welcome to use the Admirals.

I personally did not care much for the lounge and would rather pay for some of the nicer food at the terminal (or even one of the large composed salads at the Admirals) than eat the wraps and such offered at the shared premium lounge. Especially since drink vouchers are provided at the Admirals and it is often less crowded than the "new" space seems to be.