I know this has changed from the main topic a bit in that we are sort of trending towards behaviour/interruption rather than physical eye damage, but I think we can judge a bit based on the former – if something really hurts a creature’s eyes, it is not going to sit there and take it. It’s going to run or turn away at the first opportunity.
I think a lot of people take strong stances on “no flash” more because of perceived moral reasons, and because constant regular flash does actually disturb animals (a problem on tours no doubt if they are run in the same place on a daily basis).