Is this hack?

Today I saw three guys standing on the roof of a building (impossible to get there). They were all killing everybody from there, and one of them had a bazooka.

Is that some kind of hack? Jump/Fly Hack maybe, to get there.

You can boost yourself to roofs with cars.
Theres 2 ways I know.
Search for APB Epic Jump and Apb car jumping in Xfire.