It (like painting, where this is taken advantage of far more often) is actually 3D, since it involves applying one or more layers of material of nonzero thickness on a base substrate, though it can be (and often is) treated as 2D ignoring that.
But, yes, the grandparent was focussed (perhaps excessively) on advice about how to do representational drawing of objects existing in 3D space as a prelude to painting, presumably focussed on similarly representational painting.
Once the fundamentals are good - i.e. "accurately reproducing a 2D perspective projection of an anatomically correct 3D horse from any angle", you could develop your own style, make caricatures, cartoons, hyperrealism, figurative-expressionism, etc... You can have the freedom knowing that your skeleton (figuratively speaking) is correctly placed in 3D.