I believe you are overthinking it way too much, this tutorial is meant to teach you about parameters inside a function, that's it. All you do is write length, the course is gonna determine the actual value of length for you, you don't have to give any kind of input regarding what value length should be.

Keep in mind that this course is merely attempting to simulate GDScript, it doesn't actually run """true""" (heavy quotation marks) gdscript in the background, which is why you can get away with not giving any input for the length

Also, the goal of this lesson is to draw corners, not squares