https://www.bbc.co.uk/news/world-us-canada-60080080
...leaving aside the whole mask effectiveness thing, why would you turn a plane around and inconvenience all the passenger for one dissenter? Just deal with it at the destination!
An aircraft isn't turned around lightly. It can be terrifying for passengers when someone really kicks off. If they are a risk to the passengers, crew or aircraft then they need to be restrained or the aircraft diverts. That is obviously what happened.