I think, in regards to any optically guided and Mk 1 eyeball acquired IR SAM, if you approach supersonic, by the time the little Igla dude or strela operator would realize you were there you'd be gone by and he wouldn't have a hope of acquiring you.
A shilka's limited acquisition range would also be too short to pick you and react while blowing by supersonic. It's possible that Eagle is taking into account these factors.
Imagine yourself... a supersonic jet approaches you low level... you would have no clue it was there until it had blown over and was far gone... not to mention the complete disorientation you'd experience including the shockwave effects and pain of blown ear drums. :)