Either one of his staff (or maybe himself/herself) made a mistake in taking down the CC info, or the generous (magnanimous??) person called the info out incorrectly. But I'd have paid and left when they refused to pay.
I would feel for a restauranteur in this situation - a lot of them are struggling to survive at the moment. It would be a very bitter pill to swallow for him/her to have to absorb the costs of a round of drinks in such circumstances.
I understand what has been posted re the actual rights & wrongs, and the technicalities of it all, but I think you have to be fair too. You had the drinks, the bar bill needed to be settled, and it's not fair that the restaurant loses out.
I had a sort of similar experience a few years ago, though it is of no help here. I was a near penniless student, but wanted to treat my girlfriend to a romantic meal. I pushed the boat out. I was pretty excited about it, and had mentioned it to my Dad. So I spent all my disposable income for the next couple of months on a lovely meal on Valentines day. All good. I was talking to my Dad the next day and he tells me that he had rung the restaurant to give hi CC to pay for the meal, but they hadn't been able to identify which couple we were. Not sure why. But my Dad didn't go on to ask what it had cost, and give me the money. I was gutted!!!