In the interview clip posted by October Coast, Sullivan says that Affleck was committed to trying to bring forward the best possible performance, so it isn’t just Affleck saying so.
Ben Affleck says he spent a lot of time with Tom Sullivan, a blind man who was the one tasked to teach Affleck what he needed to know to portray a blind person.
Daredevil’s blindness is an important aspect of his character, both as a superhero and as a man, so it’s something that certainly requires study.