I wouldn't say it's 95% accurate. Males have these hemipenes that eventually grow long enough to fit into pockets at the tail base and some penes drop more quickly than others. They only get that hourglass look once the hemipenes are tucked into the pockets.
Some males are born "tucked in" and others can take 3-4 months. Before the hemipenes drop into place, they can hang together and look remarkably like the central bulge of a female's oviduct.
I just got "burned" in August when I bought a girl from a very experienced breeder. Between us we have nearly 50 years of herping experience and both could have sworn Titian was female. But by Thanksgiving it was most definitely clear he was a boy. Oops!!
Good thing I gave him a unisex name.