To be able to extrapolate claims to 15 million members, for a study to have any power at all, you'd have to randomly select thousands and thousands of members to poll. Basic stats class will tell you that.
He has a hunch. I have a hunch too... based of the fact that most all Mormons I know in Washington, Idaho, New Mexico, and New York (places I've lived), pull their kids from sports when they have to play or practice on Sunday. I'd guess 80 percent won't let their kids play on Sunday.