From the chart I would guess that in Y-up right-hand positive X is to the right. You look at the palm of your hand.

That way when you develop a 2d platformer you would use a standard XY coordinate system. Switching to 3d would logically add the z axis as depth and not height. A movie is usually shot that way as well.

Of course that analogy breaks down as soon as your base-game is top down. Like a city planer or so.

Anyways, with standards it's often best to just go with what most others do. So kudos to Unreal for not being stubborn.