That might be true if you're comparing the top 0.1%, but someone who played a lot of Starcraft would be miles ahead of any newcomer in both Starcraft II and Warcraft III.

Your example is like saying that a world-class sprinter would struggle to be a world-class cyclist. Yes, that's true, but the aspects that do carry over - cardio and muscle development - would immediately put them in the top 5% of the field even if they never win the Tour de France.