So? A plot twist is not defined by how common it is.
In what sense? You could have The Usual Suspects with Verbal Kint limping away from the station, and it would still be a movie. Exactly like Unbreakable. Conversely, both reveals bring new light to the plot we’ve watched up until that moment. So where’s the difference?
But both are twists.
He said she had a stomach flu. “Must have been a 24-hour thing”. I got the impression he’s lying, but maybe not.
This was a full day before the sundowning explanation, which would tie into her eating stuff that’s bad.