This is a very interesting report, but needs to be read in context. I have copied over their definition of creativity below, which seems to apply to improvisation tasks - an integral part of much contemporary training. It is also used by many contemporary choreographers - when creating a piece, they ask the dancers to produce their own phrases of movement.
This rarely happens in classical training, although I am a big fan of free improvisation to music in classical classes too. However in one ballet company where I was employed, at an audition for the following year there were over 100 candidates for 3 places. The numbers were gradually whittled down and the final dozen had to just a few minutes to prepare a piece of classical free improvisation. This was the decider! (It was held on stage and members of the ballet watched from the back of the auditorium.)
"Creativity in dance includes ideas or inspirations which begin as cognitions and/or physical impulses. The ideas, when allowed to flow, develop in many different directions to reveal something new and may lead to a distinctive dance movement or piece. The creative process involved in shaping a final product includes discovering, playing with, channelling, selecting and discarding ideas and movement material. Typically, dancers blend their own ideas, personal voice and life experience with learned art form knowledge, any tuition given, and other dancers’ ideas and input."
Like any skill, this sort of creativity improves with practice, so I can't agree that ballet dancers are less creative, just that they have usually had less time to develop this area.
Going back to the original question, I agree that a strong foundation in ballet will help in other dance disciplines.