I wasn't there, and I don't know any of the people concerned. I also understand that if you make a decision in a rush, you might think of something better later. So, I'll give the photographer the benefit of the doubt, and assume that he's telling the truth. That said, I don't recommend following his strategy if you ever wind up in a similar situation.
Transport for London have a guide to filming and photography, which explicitly says: "Flash photography or additional lighting is not permitted on any platform." As I understand it, that's for safety. If the train has just come out of a dark tunnel, then someone uses a camera flash, the driver will be temporarily blinded. So, in a case like this, that would make it harder for the driver to see the person on the track ahead. I think it would be better to find an alarm on the platform, if only to call the emergency services after the collision.