Hi,
From viewing your video, the way the additional line segments are drawn will indeed create a second and third shape (the new stroke shapes are not part of the rectangular shape.) This is normal, as the animator may wish to apply different Styles to each created shape. For example...
One tricky aspect for new Moho users is understanding that a curve is a completely separate item from a shape. A curve can contain many shapes, some stacked, some partially overlapping other shapes, and some shapes completely detached from other shapes in the same curve drawing. The result depends on how the curves are drawn, and how the shapes are created on the curve.
It may help to think of curves as the invisible skeleton of a drawing and the shapes are the visible (i.e., renderable) skin stretched over the bones of this skeleton.
Now, if you want the entire thing to be created as a single Shape, this is possible. First, create your paths with
Auto Fill and
Auto Stroke disabled. The result is a wireframe of the box with the two interior lines. Now, select all the points and paths, select Create Shape, and press enter. The result is a single shape with the same single Style applied. Here's an example...
You can create very complex single-shape drawings this way, but I wouldn't obsess too much about creating an entire drawing as a single shape. For many drawings, that's not even practical.
Anyway, this is just one way to work with Moho drawings. Moho's curves and shapes system is quite flexible, and there are many different ways to create and animate curves and shapes in Moho, and it may take some practice and experimentation to get the hang of it.
BTW, stacking shapes is a valid rigging and animation technique in Moho. You don't want to stack shapes unnecessarily, but there are times when stacking shapes can be a valuable technique. For example, you might want to stack multiple circle shapes in a single circular curve, and changing their stacking order using a Smart Bone to animate a traffic signal. (I've actually had to do that a few times.)
Since we're on the subject of the difference between Curves and Shapes, Curves can serve other functions in Moho besides creating shapes. For example, using the Follow Path tool, you can use a Curve as a motion path for other layers, or as a deformer for artwork in those layers. (In Moho 14, we have the new Curver layer that does something similar but with a more streamlined UI and implementation. Underneath the new UI, it's a Curve, though.)
Hope this helps, and have fun!