Stop and think. The guy is not deaf, he clearly knew the guitar was out of tune. Why then did ho not retune? I offer one explanation, similar (though on a grander scale) to what happened to me Monday night - I was told "you're on, two quick ones, please, we're short of time" as I came into the room, no chance to tune up. I obliged and managed to go through Emptyhanded with 2 strings out of tune somehow - and a third string went seriously out of kilter by the end of the song. I retuned after that, for the second song.
We know that the Folk Awards were live and on a tight schedule - indeed, Ralph McTell later on indicated precisely that when he said "I have been given two and a half minutes to do this introduction". I imagine everyone, Don included, was on promise to be as quick about it as they could. So he goes on, guitar is off. Dilemma - retune, or make do? He opted for the latter, just as I did on Monday (I hasten to add that this is the sum total of any similarities between us!). I can't say I blame him, you play as you find sometimes.