I believe you're probably right that it's not in the available data, at least not with 100% accuracy. But if you look in the Additional Airport Remarks section for airports with parachute operations there are *ususally* remarks that start with "A110" with some phrasing that has the word "Parachute" in it. So, to the extent that it's in the remarks section, iFly could use this information.
That being said: I looked at 5 airports with parachute operations and 3 of them had this remark and 2 didn't. Using this method would generate complaints.