[ehosca]

still waiting for the answer, how was it able to do it?

[stevenwoo]

The AIM-9X uses IR signature and it was trained to ignore US type flares which have a very consistent burn between different batches. Soviet era decoy flares are very irregular and for some reason the missile IR targetting prefers some of the Soviet flares to the Su-22 engine.

[gozur88]

That's in the article, but it's pure speculation as applied to this particular event. It's possible the pilot screwed up or the missile just failed.