Actually, it is officially allowed. IIRC, outside food used to be prohibited but that was lifted about 10 years ago once the ACs started selling hot food. I personally really appreciate the flexibility of being able to have a meal in the more quiet confines of the AC and this was something I would do regularly when I was commuting thru DFW years ago. Since the Delta SkyClubs generally have more substantial complimentary food offerings, I don't mind Delta's ban on outside food too much. But, it's frustrating when flying United and having to make a decision whether to use the United Club or get/eat a meal in the terminal. Admittedly, UA's lounge offerings have improved slightly over the past few years until COVID hit. Nonetheless, it's an AA policy that I really appreciate and I haven't seen too many issues with smelly outside food stinking up the ACs.

-FlyerBeek