As was already said the max range of the Darter is about 24km, that is from 5km altitude at Mach 1 against an effectively static target, by 20km the missile is only traveling 120m/s which is slower than most propeller aircraft and therefore wouldn’t be able to hit anything manoeuvring, the stats shown are the perfect kinetic statistics, not their practical effective range
Darter is one of the biggest disappointments, it pulls great G and its fast off the rail. Close range shots it massively over leads and corrects, It just flies straight into the ground or loses the target. They are comically easy to avoid while Aim120 and R-77 take some real effort.