Sorry but KinematicBody2d does not have a motion property.
Maybe this will help:
You can think of the nodes as predefined scripts (it's more complicated but that's a good way to understand them). These "scripts" already have some variables that are defined, in your example the rotation variable. Many of them can be edited in the inspector. They are called properties.
On the other hand each node can have another script made by you, that can contain variables that you created yourself. The scripts can also edit/set the value of the nodes properties.
Please note also that Vector2() isn't setting a type, but instead it returns a value (an empty Vector). You do not have to set the type of any variable, though it is possible since godot3.1. Vector2 is a datatype that contains a x and a y. Of course the code will make problems if you change the name of a property, because the "script" that defines the nodes basic behavior tries to find it. Changing the type of a property leads to trouble to, because godot is expecting a certain type.
Did that help you? I hope so. Feel free to ask more questions if not.