Correct, but based upon that receipt, I would (a) show the receipt to the agent and try to get it honored (unlikely, but worth a try), and then (b) file a refund request with UA after the fact. If they refuse the refund request, I’d file a DOT complaint.

The DOT requires that the receipts be correct. Even though we all know that the correct value is 1 free bag, OP was offered two, and UA needs to honor that (and I suspect they will, begrudgingly, after a DOT complaint).