The theory is that the term Augmented Reality has too much associated baggage accumulated over the years from implementations that don't do realistic 6DOF.
So they are trying to say - approximately - that anything that doesn't have depth is AR and anything that does is MR.
Graeme Devine made this distinction by throwing shade at the iterations of mobile AR from previous generations that didn't have SLAM or any kind of tracking in a recent talk [1].
In reality MR was defined by Paul Milgram in 1994, and does well to describe the spectrum that encompasses Augmented Reality, Augmented Virtuality and Virtual Reality.
[1]https://www.vrandfun.com/magic-leap-provides-updates-dice-su...
https://www.youtube.com/watch?v=3UQK_ksU3kc http://www.horizonirs.com/vr-ar-ir-mr-vw-hmd-hud-whaaaat-2/