I think the difference is that mixed reality is assuming you will mostly stay in one place with defined boundaries, and the device doesn't take in a lot of spatial information about where you are. But it has video pass through so you are aware of your surroundings. Augmented reality is similar, but it is more like trying to illustrate on the world around you (put an object on the real table in front of you). AR is draw on your environment as you move around. MR is like VR with pass through.

I think the distinction they have is helpful. They call it mixed reality because when people say AR, they assume virtual stuff overlaid on the actual real world stuff (like with glasses). Quest Pro isn't that, you don't look at the real world through lenses, you are looking at what the camera feed of the real world in front of you displayed on the "screen".