Not quite true. You can use Thunderbolt 3 over USB C, but my guess is most USB 3.1 ports out there will not support Thunderbolt 3.
BTW, this is probably the real reason the Retina MacBook uses USB C. The form factor was designed for future technology - ie. Skylake with Thunderbolt USB C, not current tech.
I've said many times, Apple often designs its new hardware for the future, which means that the first interation of any new Apple product is often quite gimped in some way, only to have that gimp removed in the 2nd or 3rd iteration. My main other hope is that the next Retina MacBook also gets a second USB C port (and a faster CPU).