Only reason why I believe it to be true is that Link has to open the clouds in order to gain access to the surface. Otherwise he could visit all three provinces from the beginning.
I thought it just meant its impenetrable to loftwings. You could be right though he actually has to open a area in the clouds to jump to the surface, but it also could be just a area made to get his bearings instead of just randomely jumping and ending up in weird spots way off course of were he should be.