Why surprised? Atlanta is predominantly a Skyteam hub with most passengers flying out on Delta or their partners who will be using the DL lounges

BA maintains a single daily flight to London.

They don't have access to DL lounges. Their Oneworld partner AA has their own lounge as you point out but it's in the domestic area of the airport so not easily accessible to you. This is why the Club @ ATL has been selected as BA's lounge. There's not exactly a lot of choice in the matter.

If you want nice and quiet and the Club @ ATL just isn't cutting it, you can always check out and throw some money to the Private Suite
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I guess a lot will depend on how important the nice and quiet aspect of the airport experience is to you.