Think of MEB as a set of abstract parts with default implementations. If you want to change the whole vehicle, you only have to change certain parts. Make these pieces bigger, these pieces smaller, etc.

A true skateboard would be a static implementation of the frame and propulsion system, with defined interfaces for the "top hat" (industry term for the body of the vehicle).

A car platform is a set of shared components and design concepts, but not an actual physical thing (a "skateboard"). Same platform cars can have different engines, wheelbases, suspension types and setup, transmission types and so on.

Most modern cars use the monocoque/unibody type of body, where the body and frame ("skateboard") are combined into one, because of weight and safety (it can be made to bend in a more controlled manner, usually with different type of materials bonded/welded together). The frame+body is better because it's easier to repair (many unibody crashes are not economical to repair because a proper repair to the same strength as the initial car would need dismantling half the car).