There is no proof that being able to do 2D art at any level means you will be good at 3D Art.

I understand why they want a 2D Portfolio, its to show you have an understanding and background in Art. The reason I think they are not interested in 3D art, is the fact they are going to teach you and it is not a requirement to have any 3D knowledge or experience.

But still, there are ppl out there who cannot do 2D art, but do excel at 3D Art.