Emissions

This question is not necessarily in regard to my own personal vehicle. I really want to understand the logic behind the following scenario, I would really appreciate an in depth response:

There was this recent video I ran into of this guy who claims that his Ford Explorer gave him 13.6 miles per gallon highway and how he manages to get 56% better gas mileage because of this "trick", as he advertises. He shows these tiny hho generators that will feed the combustion chamber Oxygen that has increase his 30% fuel burn to "100%". Giving all these figures, 56% improved mileage highway and some other percentage city. I am relatively new to understanding how cars work, but this video has taken over the internet like wildfire with its sensational title and this fast, smooth talking guy. I am not entirely discrediting his claims but I feel skeptical especially since there are amazing engineers who are constantly finding ways to lower emissions, why would they hide these techniques as the title of this video very clearly suggests? Wouldn't the hho generators in the video not be able to produce enough hydrogen to change the fuel mileage? My question: is this claim legitimate? If not, please explain in layman. Thank you so much.