Does that mean Augmented Reality is a form of Mixed Reality? Or what's the difference?

This distinction is relatively fine.

Augmented Reality displays information overlayed on a real view. While Mixed Reality (MR) allows the user to interact with the actual environment.

This usually requires a good understanding of the actual environment and can currently be experienced with Augmented Reality Headsets such as the HoloLens or the Magic Leap. Smartglasses are also a great tool for MR.