Not all 2D animation is completely hand drawn, and the ones that are computer enhanced can still be animated properly…
For example, the current generation of "My Little Pony" is animated in Flash; sure, they still have to draw the initial sprites, but once they've got the various sprites, they can fully animate the characters relatively easily. And for that matter, it's not even like they have to draw the entire characters' sprites for each frame, they have a pool of sprites for character's mouths, a pool of sprites for character's eyes etc. so they can mix and match things to get whatever facial expression/pose/whatever they need to without having to do any extra drawings…
The point is, by using a technique like this, they could easily make a high-quality Sly film that's in the original art style, and like they said, they're not making it 3D because it's "easier" they're making it 3D because it's apparently more popular…