PI can only be called when there is an intended receiver. There cannot be an intended receiver if the ball has not left the QBs hands. If a defender interferes with the intended receiver before the ball arrives then it is a penalty. Likewise if the intended receiver interferes with the defender before the ball arrives, it is OPI.
Illegal contact is when a defender interferes with a potential receiver before the ball is THROWN. Likewise if a potential receiver interferes with a defender before the ball is thrown it is an illegal block downfield.
PI prevents an intended receiver from catching a pass, therefore it is a foul. IC prevents a receiver from even becoming an intended target in the first place. That is the difference.
If the ball is uncatchable it becomes difficult to prove an intended receiver. Also, the whole point of PI is that it prevents what could otherwise have been a catch. If the ball is not catchable then the interference could not have prevented a catch and therefore no unfair advantage was gained by the defender. Although the argument could be used that if the ball is not catchable then the defender has even less excuse to commit the foul in the first place, the logic behind the rule still stands.
I did not see the play in question so I cannot give an opinion on it, but I hope I have shed some light on how it SHOULD be called.