An INFIELD FLY is a fair fly ball (not including a line drive nor an attempted bunt) which can be caught by an infielder with ordinary effort, when first and second, or first, second and third bases are occupied, before two are out. The pitcher, catcher and any outfielder who stations himself in the infield on the play shall be considered infielders for the purpose of this rule.
When it seems apparent that a batted ball will be an Infield Fly, the umpire shall immediately declare Infield Fly for the benefit of the runners. If the ball is near the baselines, the umpire shall declare Infield Fly, if Fair.
The ball is alive and runners may advance at the risk of the ball being caught, or retouch and advance after the ball is touched, the same as on any fly ball. If the hit becomes a foul ball, it is treated the same as any foul.
If a declared Infield Fly is allowed to fall untouched to the ground, and bounces foul before passing first or third base, it is a foul ball. If a declared Infield Fly falls untouched to the ground outside the baseline, and bounces fair before passing first or third base, it is an Infield Fly.
Rule 2.00 (Infield Fly) Comment: On the infield fly rule the umpire is to rule whether the ball could ordinarily have been handled by an infieldernot by some arbitrary limitation such as the grass, or the base lines. The umpire must rule also that a ball is an infield fly, even if handled by an outfielder, if, in the umpires judgment, the ball could have been as easily handled by an infielder. The infield fly is in no sense to be considered an appeal play. The umpires judgment must govern, and the decision should be made immediately.
When an infield fly rule is called, runners may advance at their own risk. If on an infield fly rule, the infielder intentionally drops a fair ball, the ball remains in play despite the provisions of Rule 6.05 (L). The infield fly rule takes precedence.
Bolded portions mine
A theory of why the ball was not caught is not necessary. If this were an infield fly where the SS didn't have to move at all, and he just happened to drop the ball, everyone would say "wow, that sucks...but that's the rule." The reason this is an issue is because of the location where the drop occurred, which is outside of where most (all?) of us think when we think "infield fly". However, based on the rule, IMO there's no doubt the play is technically an infield fly.
I don't know if the "comment" section above is an official part of the rules, but since I pulled this from the MLB website, I assume so. The fact that it mentions that an outfielder can handle the ball suggests that a fairly deep ball can still be an infield fly. Further, it is stated that the umpire's judgement governs the call. Arguments about "ordinary" effort seem silly to me; any MLB SS should make that play with relative ease.
If you want to consider it one of those rules that's never called, I suppose you can make that argument. But can any of us ever remember a situation where we said "well, that ball was technically an infield fly, but that call is never made..."? I can't. Baseball is inconsistent with that sort of thing, anyway - for example, it's ok for a 2B to make a phantom tag of the bag during a double play, but it's not ever ok for a runner to miss touching a bag.