It wasn't elaborated on because it wasn't part of the core point of the comment, which was someone claiming falsely that to write Common Lisp you needed to use CLOS, which you absolutely don't.

You wouldn't go into why JavaScript is bad to point out that you don't need to write JavaScript to make a web page. You would just note that JavaScript was a late addition to the web and wasn't the first language usable on it.

Plenty of people were writing CL before CLOS existed.