This is incorrect.
The program was to chose the next standard issue variable fighter, and the YF-19 won that competition.
The YF-21 was redesigned (and redesignated VF-22) to be a heavy tactical fighter for special operations, not mass production.
VF-11 --> VF-19
VF-17 --> VF-22
However, mass production of the VF-19 never really seemed to be picked up, whether it was because of expense or he difficultly involved in flying the things, which is why Frontier has the standard issue fighter being an upgrade of the VF-17.