>>264355490
It is legitimate exercise m8. It qualifies as fairly vigorous cardio especially at the higher difficulties. Comparable to HIIT type training. 
https://vrhealth.institute/portfolio/beat-saber/

It's also just really good exercise in general because it's fun and if an exercise is fun you'll do it more. 30 minutes of it a day would keep you in good cardiovascular health.
https://www.mayoclinic.org/healthy-lifestyle/fitness/expert-answers/exercise/faq-20057916

Play beat saber.