He's saying give the security guy your burner phone as a faint. Then the security guy has no reason (and no excuse) to metal detect you and your real phone. Then you use your unpouched real phone to record the concert.
But the security guy is also looking for guns and knives and drugs - given the god awful murders of musicians recently isn't that a decent reason to search until "no more bleeping"?
That same thought came to me when I read the article, but from a different angle: I have a work phone and a private phone, so I could just give up the work phone and see if that's enough for them.