What I think onlookers are having difficulty with is believing that using your other open eye which sees the whole scene the brain is able to superimpose the scene over the black viewing screen and you can then move the spot to whatever you have chosen in the scene as you metering spot.
On the one hand provided your brain in able to superimpose the scene for long enough to aim the spot and secure a reading then it should work fine. It almost seems too easy to be true but on the other hand none of the three reviewers seem to have any difficulty in doing what the inventor says you need to do.