I would be quite interested to know how you managed to measure that and get a distance of 2.5 m, since that is the approximate tail rotor ground clearance for the
AW-101 Merlin, which is ~1.3 m taller than an NH90 (6.62m vs. 5.31m). The best (i.e. greatest amount of tail rotor ground clearance)I have been able to work out is only perhaps 2.1m of ground clearance, and this would be if the helicopter is on level, even ground. This is based off the max height of ~5.31m with rotors spinning, and a tail rotor diameter of ~3.2m.
I would also hazard a guess that one did not really pay close attention to the content of the embedded Youtube videos, as they too suggest that the tail rotor clearance is a hazard. In the first video, at the 1:05 mark, troops can be seen disembarking from the NH90 via the rear ramp. It should be noted that the troops are streaming off the ramp and away from the NH90 on the side opposite where the tail rotor is
In the second video, there are several things worth noting. One of the first is that after the German Heer NH90 TTH had landed and stopped, the pilot does a brief walk along the side of the helicopter where the tail rotor is and at 3:43 he touches an exterior conduit which contains an antennae. That conduit is just about level with the pilot's forehead and is only a few cm below the bottom of the tail rotor arc When looking a portions of the video of the NH90 in flight, other features of the helicopter like the rearmost window on the port/left side, is also just about level with the bottom of of the tail rotor's arc, and at 2:51 the pilot appears to be standing higher than the top of that window. Also, at 4:26 when troops are exiting the NH90 via the rear ramp, a pair of troops position themselves directly beneath the tail boom and crouch down, whilst the rest of the troops disembark down and to the right rear of the ramp, again on the side opposite of the tail rotor. This once again strongly suggests that the tail rotor would present a hazard when spinning, particularly for troops kitted up in their battle rattle and/or at max height.
Not sure about German pilots in the Bundeswehr, but the max height of USMC personnel is 6ft 6in or ~1.98m. I doubt the German pilot would have been that tall. This once again reinforces the notion that the tail rotor presents a hazard while in motion which would limit the ability to use a rear ramp. This would be particularly true if the helicopter was not landing on a flat, level surface, or was doing a rapid landing where the tail of the helicopter was angled down instead of level.