I think that was his point
Although it wasn't very clear.
He did say that if you can remove the phrase within the commas without changing the meaning of the sentence, you have used the commas correctly. What he didn't do was provide the corrected sentence for his second example: "Hey, don't you think he's a dumba$$?"
Given that most people don't use 'dumba$$' in their everyday writing, it was a poorly-chosen example.