Whats subjective about the leading English dictionaries, almost to a word, agreeing on the definition of something?
World Class is clearly 'to be amongst the best in the World in that given field'....
...a world-class restaurant, doesnt need to be the best in the World, but it needs to be amongst the best
...a world-class opera singer, doesnt need to be the best in the World, but he/she needs to be amongst the best
...the same goes for footballing left-backs.
For what its worth, it looks to me like Marcelo is the best left-back in the World, and would be my pick for my 'World XI'...however, Robertson on current form and the current level he is performing at, should be considered as World Class...because there aren't many around that are better than him.