Was Snape lying about the Confundus charm? (Prisoner of Azkaban)

Currently reading through #3 for the first time and after Harry "attacked" Snape in the Shrieking Shack, Snape claims that the trio were under a Confundus Charm from Sirius and that's why Harry expelliarmused (if thats a word) him and the trio believed Sirius was innocent.

Was Snape being serious that he thought they'd been cursed or was this an occurrence of him *protecting* Harry for Lilly's sake? Because I feel that a wizard like Snape would actually be able to tell if people were cursed.