The argument about the hockey program you make having no transfer to anything else is simply false. Gladstone points out that this "problem" if we should call it such happens when there is differentiated instruction that begins at too early an age. Basically kids who are older are more developed and a little higher achieving because of a developmental advantage not a true intellectual advantage (or physical for sports). This happens in our education system. It is probably more pronounced in other countries that follow a model where younger kids are separated into tracks that determine their future literally at too young an age. If your child is on the cusp of being able to enter kindergarten early or wait a year, some parents think starting early is going to give that advantage. Clearly it isn't since that child will developmentally be behind nearly a year compared to the rest of the kids. They won't be recognized as being more intelligent even if they are because compared to their older peers they are average or even a little behind. I think this absolutely is true. Gladstone pointed this out on the Tims science assessment when age differences were taken into consideration.
Can someone achieve what Einstein did by simply putting 10,000 hours into math? No, not possible. But can someone not putting in 10,000 hours produce such work. Not possible either. One has to understand all of the current work to prove or disprove such.
I wouldn't call this book junk science by any means. It is simply a perspective with some science. We also know for example that those that make unique contributions to a field tend to be anti- authoritarian in nature, strong willed in their own opinions (they don't accept things just because they are told so) among other characteristics. Certainly, however, the environmental advantages and the time factor play a significant role.
As someone who is quite well read on much of the science of gifted and talented, this point of view that Gladstone presents is quite underappreciated and welcome.