Hi, yes ecoinvent uses a simple model, but you do not need to calculate the distance twice as the truck in reality will rarely drive empty on its way back. So the return trip is not your responsibility any more, so to say, and it would be just 100t*10km= 1,000 tkm.
Edited because of your comment below: It is 100t*10 km + 0t*10km = 1,000 tkm.