Hi,
according to the battle simulator at least the leaf-lift is not a ground unit.
http://prntscr.com/l1gxhoAfter this I got curious and added everything the simulator had to offer with this result:
http://prntscr.com/l1gyayThere are 3 buildings that seem to be no "ground units": Alliance Station, Quay, Leaf-Lift.
Since this all are buildings that literally have to stand on the ground I think this is a bug. They should count as ground units and therefore could be destroyed by guardians only.