Small but important distinction - a twist ending doesn't happen near the end of a film, it happens near the end of a story. As per Google;
A surprise ending is a plot twist occurring near or at the conclusion of a story: an unexpected conclusion to a work of fiction that causes the audience to reevaluate the narrative or characters.
Being as the reveal about Snape comes towards the end of the overall story (that is, the over-arching story of the franchise rather than the individual stories of each film/book), we can safely say that yes, it does count as a "twist ending", as it is a reveal that causes us (the audience) to reevaluate the character.
However, it should be noted that as far as "twist endings" go, it's a rather tame one in comparison to other famous twist endings, such as The Sixth Sense or Planet of the Apes, each of which contain much bigger "twists" and in some way came to redefine what we consider "real" twist endings.