Fairly said, but there is a nuance - I was speaking about mechanics in a different sense than rules specifically. I was speaking of mechanics as the built in 'crunch' of the game as provided. If the mechanics of the game do not serve his story, he makes new ones.
As you mention, they make new subclasses, he makes new monsters, he makes up drugs, he takes parts of two subclasses and finds a way to merge them in a villain, he creates entire schools of magic, he takes them on weird journeys into a mind, etc .... all of which are not based upon the existing mechanics as laid out.
The mechanics are not a menu for Matt Mercer. They're just an option. He peeks at the established options to see if they do what he wants, and if not he modifies them or makes up new stuff to tell his story.