Actually, "de facto" is in contrast to "de jure", meaning "in law" or more generally "officially". If they leave the planes in the air, then whatever the offical reason, they are de facto asserting that there is no (significant) problem. (Grounding them on the other hand could just be considered a excess of caution, so at most it asserts "We aren't sure there isn't a problem.".[0])

0: Which action, if either, is definitive might differ if the stakes weren't inconvenience vs death, or if the FAA openly didn't care.