He is wrong.
Notices on Verizon are optional, and are not automatically piped through like they are on GSM.
I opted out on my Verizon phones the first day they enabled them.
Amber and emergency notices are optional on AT&T too. It’s basically a system-wide text, I see no reason why GSM or CSDA makes a difference.
In the unlikely event that a Presidential alert is issued (by Obama or whomever succeeds him), you’ll get the message just like any AT&T subscriber will. It’s an FCC requirement.