I always just took it as him seeing the flash after it had already moved i.e. seeing light after its already reached your eyes. Regardless, he had to set things up so that he knew that path it would travel ahead of time for Silver Chariot to cut it because he says he can’t see it. Not exactly something he would bother doing if he could. The entire fight of Hanged Man was setting things up so the beam of light it is could only travel one path in order to beat it. That’s the only way how Polarneff was able to get him the two times he tagged him. Silver Chariot is FTL but limited by Polnareff’s kinetic vision and there are a few characters FTL on their own like Kars, Joseph, and Rohan but Polnareff himself isn’t since the whole fight with Hanged Man wouldn’t make sense otherwise. His Stand is more than fast enough to tag it as it’s moving and he wouldn’t need to waste time controlling and predicting where the beam travels if he really could see and know anyway.