That jumped out at me too. I can't imagine a good Texas boy like Don using the word mate. Agree that maybe he was localizing it. I have a friend who lives in England that I travel with, a boss who is originally from Dublin, and I probably watch way too much British TV. I find myself unconsciously using words and phrases I've picked up that I know are very Irish and English. But it does crack me up thinking of him saying that in his Texas twang
And no, it's not regularly used in the US, that I know of. At least not in NY.