If it's iodine, then it's antiseptic, right? I don't see why they would use it specifically, though: AFAIK, they test for STDs regularly (if not for every shoot?), so it looks more like "just in case" measure. Maybe something "strong" like that feels better and more "cleansing" than regular mouthwash to get rid of all the cum etc.
As a side note, maybe if you see them using it, it might be a way for us to make sure that the cum-shots are real (or it might be a part of an even more elaborated way for the maker to convince us that it's real... the conspiracy thickens)