What makes it not terrorism? Because the target was government-run facilities instead of civilians, or something else?
Pandora's box
A state military attacking a perceived threat to the national security of that state (while at the same time doing its damndest to make sure nobody knew about it) is pretty clearly outside the definition of terrorism. It fits squarely into espionage / warfare.
None of the terrorism boxes get ticked. It wasn't a splashy, overt thing meant to instill fear. It wasn't carried out against emotionally-charged targets attempting to incite, nobody claimed credit, etc.
Everything adverse that happens is not terrorism. The term has kinda worn itself out, which is bad, because that word invokes a whole bunch of executive power shifts.
Also probably a bit of, because we did it instead of it being done to us.