It's not about the airframe, it's about the radar. The Kh-66 (and the RS-2US) could only be fired by MiG-21 variants that carried the RP-21 Sapfir. The official Polish bis manual (factory designation izdeliye 75A, equipped with the RP-22SMA) supports this - neither the Kh-66 nor the RS-2US are listed as available air-to-ground weapons. These can be fired by any Fishbed model that isn't an S, SM, SMT or bis.
EDIT:
You're right Kang, they only added it to expand our MiG's weapon arsenal, not because it was used in real life.