It's not really absolutely required as much as it is ideal, in my outdoor grows at night a neighbor has a porch light that shines from a distance, and I know damn well it's not 100% darkness especially with the moon and stars.
Even if there is a tiny little light leak inside a tent, there's no guarantee it will go hermie. But we strive for ideality
Absolute darkness is not required. Consistent darkness pretty much is. Plants are able to deal with cycles like that of a full moon to new moon, and back, as well as other less natural sources of light during their dark periods. I've never seen a study that shows how much light is needed to trigger problems.