does that mean that when drawing a mountain for instance we don’t draw the dark side of the mountain but only shadows cast by rocks or other objects on the mountain?

That is correct. The "dark side" of the mountain is the result of form shading, where it's the orientation of the object as a whole that makes it lighter or darker. As explained here, we are not applying form shading in our drawings for this course, except for the crumpled paper you would have handled in Lesson 2, which is a special case, as explained in the homework section for that exercise as quoted below:

For the crumpled paper, you're actually not going to worry about cast shadows - instead, its purpose is to get you used to working with bold, clearly defined black shapes, rather than being timid and using hatching. Your crumpled paper study/gradient should be made up of clearly defined, distinct black shapes and white shapes. We find that despite breaking away from cast shadows, this helps students become bolder and more confident, which then yields better results for the other two textures that follow.

Cast shadows are always the result of one form blocking the light from reaching another surface. It's the specific shape of the cast shadow that establishes the relationship between the form casting it and the surface receiving it, and that's what we want to be focusing on here. You can also read more about this, and get more clarifications from this section of reminders.