Max_score is the same things as for the capa problem. module_score is current score in database.
I think at current state it is safe to remove this check. There is some context:
It can be the case, when student has already received grade for a video, but after that teacher has changed graders settings. 
Now, video player has two score values: 0 and max_score. No intermediate value is stored.
It can be the case, that in the future other grade values for video player will be stored. Like 0.5 or 0.7.
So it was designed not to change student score, whatever graders will return, if student already has received maximum score.