Emersive vs augmented VR
There are basically two type of VR: augmented and immersive.
Immersive VR takes away your normal eye-to-real-world optical stream and replaces it with a video stream.
Augmented VR is more of a heads-up display that paints images on top of normal sight.
In the photos Mann is clearly wearing immersive while the Googleboy is wearing augmented (since you can see his real eyes).
Augmented VR suffers registration issues (ie. it is challenging to sync up the projected overlay with natural sight) but it does not interrupt normal sight.
Mann's immersive VR does away with registration issues, but shifts your view.
The brain is very adaptive. People with glasses manage to cope fine with changes when they take glasses off (or put them on). My glasses have both magnification and realignment (ie combination of lenses and prisms) and I can happily take my glasses off or put them on while walking/driving etc with no disorientation.
I am sure Mann is making more of this than he needs to and is defending what he feels is his turf.