I do agree masks on open platforms with no one around is absurd but then one might say so is banning smoking on open platforms with no one else around.Sadiq Kahn is not a national politician and is not in government despite the fact that he would like to believe he is.
Surely you can see that mandating that people wear masks on empty open platforms is absurd and is in no way about health but control.
I am in agreement with @3rd rail land that no one is going to enforce this even if the byelaws are changed as someone only has to say I'm exempt and the enforcer then has to lose all interest.
Why does Sadiq make up rules that are contrary to Government policy? Because he can!
Why do some people claim to be exempt to bypass those rules? Because they can!