You do have a gift for chasing your tail and concentrating on irrelevant information.

Under U.S. Department of Transportation rules the marketing carrier of the first segment matters for baggage allowances to/from a U.S. destination. Don't expect BA.com to display your AA luggage allowance properly, but the check in system should handle it. Do expect the AA ticket receipt to show the correct allowance. Do expect US to recognize the entirety of your ticket and provide the correct allowance: it's programmed into the computer - they don't want to waste agents' time with this stuff.

Yes, it's regrettable that the Twitter team gave you incorrect information.