A twist on the old if-a-tree-falls-in-the-forest riddle: If a hidden gesture exists but nobody knows it, does it really exist?
But to answer your question: It's hidden because it doesn't have a graphic representation on the device itself. Nothing on the volume button indicates that it's somehow a hidden slider.
The whole point of a good UI is you shouldn't be reading a manual and watching videos to do X, Y and Z.
Apple used to be a pioneer at things like this.
Sad things have come down to this.