The article says "Smalltalk classes are powerful because they themselves are objects".

This is true of CLOS (the Common Lisp Object System) as well. Every class is also an object. Which itself is an instance of...a metaclass. Which is also an object. Which you can introspect and modify at runtime.

One can get wrapped around the axle by modifying such things casually, but when you really need it, it's quite handy.