Thats a good point. Who is telling you that its not AT?
Now that I think about it, I have been on on AT where I concentrated on the hill walking but mountain biking has been one of the other activities.
IIRC the adventure training world has a list of activities that each service sponsor and are considered official. The RN stuff involves getting wet, the Army stuff involves a low-level beatsing on the ground (and in-land canoeing) and the airforce stuff involves getting airborne.
I think that walking/trekking on trails is considered adventure training. In the past mountain biking those same trails was considered to be sport. In my view mountain biking involves similar navigation, greater speed, more risk, more fitness and a small mechanical element. I would think that mountain biking would tick as many boxes as walking/trekking. From what I have heard the Army is moving towards off-road cycling becoming a recognised adventure training activity.
I am sure its moving in the right direction. There is a thread on ArmyNet about this. Some of the people there will give a better answer.