So was Bill Walton.
correct me if im wrong, but Walton was a vegetarian not a vegan.
Regarding Kyrie, based on the very limited info available on the SI article he didn't go 100% vegan and it is not for life.
Vegan is a life choice, and not an easy one. It's not just eggs or milk, but even chewing gyms or wearing clothes made of leather.
Needless to say, this is hardly advisable long term for professional sportsmen.