we can't do otherwise than following the shape sometimes, same with the bricks ?

This isn't actually true. I've drawn a quick example here. As you can see, every textural mark is a shadow shape, and as such they do not have to surround each brick completely, nor each strand/tuft of hair. We do still use outlines around the silhouette of the form, but it's the internal details that we use implicit techniques on.

Now, you're not wrong that there are going to be areas where the shadows expand enough that they'll surround the entire textural form, functioning similarly to an outline, but that's normal. The point is however that they're still shadows - we're not using line. To force yourself to use cast shadow shapes, this two-step process can help, if you make a habit of using it for each and every textural mark.

As for whether or not you should redo the exercise, you can certainly explore it as part of your warmups, or you can choose to work on the optional 25 texture challenge, but when your lesson is marked as complete, that means you are cleared to move forwards, and that is the recommended course of action. We do not expect students to have a full grasp of how to use texture after this lesson - it is merely an introduction to the concept.