Only ADS-B Out (which refers to an aircraft broadcasting its position and other information) will be required in the near future. ADS-B In (which refers to an aircraft receiving the broadcasts and messages) remains optional.

Also, due to the common airport design of runways and parallel taxiways, it isn't uncommon to fly either near or over waiting planes on final approach, so this may not have been cause for alarm even if they were equipped with ADS-B In.