It does not work just fine.
You're not making LISP case-sensitive there. You're forcing the reader to invert the case of every character at read time. This creates the illusion of case sensitivity while silently breaking things in non-intuitive.
For example, readline tab completion is broken by using :invert. And if someone else wrote code using the default :upcase and used any capital letters that code will be broken if it is run in an environment using :invert.
An example:
(defvar openFuture 't) ; works fine
(setf (readtable-case *readtable*) :invert) ; no problems yet
(format 't "openFuture == ~a" openFuture) ; variable openFuture does not existWhat actually happens there is the first line creates a variable called OPENFUTURE not openFuture. Then we switch to :invert mode. Then the third line tried to print a variable called OPENfUTURE.
The supposed freedom of LISP here actually creates an environment in which everyone is forced to pretend the language is case insensitive.