He is kinda correct. If you can stack the courts, you can control what the law means and even rewrite it without anyone having any way to stop you.
Which is why FDR tried it in the 30's. SCOTUS told him that a lot of the New Deal legislation was unconstitutional, so he said "Let's see what the court says when there's 19 of you, and I get to fill the 10 vacancies now."