nono, I am talking aboiut two different keywords.
IS checks for class, returns true or false :
if myvar is RigidBody :
AS is added to variable to force compiller to bahave, like this variable is of certain class. If You wrote :
mywar = get_node("boom") as RigidBody
compiller will finish your sentences, just like myvar would be of class rigidbody, even if in reality it is another class.