Harsh!
You are confused because we routinely talk about dimensions incorrectly. The 4th dimension is time (and space because space is also time). So technically all number plates are 4 dimensional.
What they are trying to describe is the difference between a flat, two dimensional appearance, a two dimensional drawing with depth / shading that therefore looks 3 dimensional and a plate which is actually 3 dimensional in appearance as the letters are raised.
My cardigan is getting very warm this evening.