What I don't understand is why one would opt for an IR (heat seeking) missile, over a radar homing one? In this case the goal - I'm guessing - was to pop the ballon and not damage the payload more than necessary.
Any heat emitted must have been from the payload, so an IR seeker might hit the part you want to keep (the only part emitting any heat at all).
If you believe that the payload was a sophisticated signals investigation platform, sending a radar-homing missile towards it potentially gives your adversary access to information about your missile's capabilities.
I guess. Likely the F-22 radar itself is a better kept secret than the AMRAAM which has existed for a long time and in semi-recent versions in places like Pakistan. So that could be a reason: firing a radar homing missile requires using the plane radar (?) and you'd prefer to avoid that if possible.
