The AIRS has no external references beyond the initial position set by the ground station before the missile was fired. That was why AIRS had to be so incredib

The AIRS has no external references beyond the initial position set by the ground station before the missile was fired. That was why AIRS had to be so incredibly low-drift. The point of the comment in the writeup is that the DoD found that there were almost no cases where at least some sort of external reference wasn't available and thus there was no point to having such an expensive, low-drift INS.