I had to side with Bronn on that one. He knew he had no chance at beating the Mountain, so what was the point is going to fight him? Why walk to an automatic death? Bronn was no (right-handed) Jamie - he was just a sellsword, nothing more. And he told Tyrion that from the jump. I'm sure he was devastated for Tyrion, but I would argue Bronn didn't owe him anything.
I agree. My point tho is that Bronn has never been about nobility, honor or anything that would prompt him to be selfless. His talk with Tyrion about the Mountain tells you everything you need to know. He asked Tyrion why he should risk his life for him when Tyrion would never do the same thing. And then told him the only way he would bail him out was if Tyrion could make him a better offer.
Jamie literally sent him on a suicide mission to man the crossbolt. On what planet would that prompt Bronn to act selflessly on Jamie's behalf? He did it because he was being paid to; dude who was trying to argue otherwise just don't see it.