Hi and welcome to the forum!
Yes you are correct - movement is in relation to the 2D plan.
If in 3D you are looking at the model from the front (eg in 2D placing a new camera at the bottom of the screen and dragging to the top) then the movement of Nudge will be correct - left arrow will move objects to the left and right to the right, while down arrow will move them towards you and up arrow away from you. In 3D however, you can view from any angle, so if you have a 3D view from the top of the screen then the movements are reversed which can be confusing to say the least!
Your suggested directions would only apply to the specific angle you were viewing at that time. Move the camera to the opposite side and they would be reversed.
Generally it is easier to use the Nudge in plan view as you have found.