He's not wrong. Look at the amendment that banned alcohol. It was repealed through another amendment.
The Constitution is meant to be updated and changed with modern times. What he's meaning to say is that despite Republicans (who don't even understand the fucking document) believing the Constitution is set in stone, it's actually meant to be changed regularly.
I mean, he's right though? The point of the Constitution (and amending it) is to be able to change it so we can improve how we are governed. It's not inviolate and sacrosanct, it's supposed to be living.
Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.