This is correct. You would need several hundred or even thousands in order to prove that the tail bone theory is a solid method for sexing them, but if it were a viable method I'm sure that those who study these frogs on a day to day basis would have already know n and hence forth published the information. I have never seen any articles or studies that mention the tail bone as a method for sexing.
I still don't see it as a viable method at this point.