It’s true! Dancers who begin their study with solid training in ballet are preparing themselves for maximum versatility for other dance genres. And here is an article on this very topic.
In my opinion though, it is important that other forms of dance like modern, hip-hop and tap be introduced soon after an intermediate level of ballet is reached by the student. We can’t all be Alex Wong! – Most dancers have to learn how to use their ballet training to explore new ways of moving in space. But like the article says, it rarely if ever can work the other way around.