This seems like extreme nonsense. How light would you have to exercise to only breathe through your nose? I had an ex gf that tried to do this when we would go running and it was a disaster.
If you commit to nasal breathing for exercise as a constraint, it does force you to modulate your exertion while also increasing your CO2 and developing, according to the Buteyko folks, a new baseline for respiratory health and capability.
If you're running for your life from a tsunami, by all means mouth breathe. If your purpose is maximum exertion, of course mouth breathe. But that's not the only possible purpose of exercise. It can also be about respiratory training. Nasal breathing becomes a natural guideline/modulator for long term improvement in that regard.