Very few NYCB members started at the school at 8-10, like at Paris Opera Ballet, the Mariinsky Ballet, Royal Danish Ballet, and Bolshoi Ballet. Those programs are hardcore professional dance training, and the physical vetting process is beyond anything in North America and most schools across the world. Even then, if you've ever seen footage of the kids being selected, they aren't starting ballet from scratch, so "pure" is relative.
Almost all of the students at SAB come into the Professional Division as teenagers, which means they've had extensive training elsewhere. Very few are NY Metro locals who started at SAB as children and made it into the Professional Division, let alone into the company. They often come from other schools, even if they're local to NY Metro, they take the summer program, and from there are invited to take the year-round program. Even if they're in the year-round program, they often do summer programs elsewhere.
I'm not even sure the few dancers that started at SAB as children and made it to the PD and NYC, like Peter Boal, Jennie Somogyi, Michael Byars, and Judith Fugate, were "pure" SAB products, in that any of them might have done summer programs.