All towbar-less tugs I've worked with need to pull/roll the aircraft onto a lifting plate which then lifts off the ground lifting the aircraft with it. So how is the nose supposed to be rolled onto this lifting plate if the aircraft cannot be rolled with the parking brake set??? Perhaps you havn't noticed the roll because it is subtle but it's there, unless there are new towbarless tugs out there that somehow levitate the nose tires off the ground without the need to roll the aircraft that I haven't seen before, which is quite possible.