It's not clear to me why domestic vs. international matters with respect to Texas law.

To the extent that Texas (or local DFW) law is relevant here, it's with respect to regulating local alcohol service. That regulation would apply regardless of where the plane is going.

​​As a practical matter, the impact of local law (such as whether the airline has appropriate authority to serve alcohol) can differ between airlines, but the applicable law would be the same.

Similarly, the point raised up thread about duty paid alcohol could be in play. If BA had an appropriate liquor license, but didn't have duty paid liquor, the license wouldn't help. But that wouldn't be because Texas law prohibited BA from serving.