Are lockers really that secure? Similar documents advise against leaving laptops in hotel rooms or cars, even if locked, because they are easy to get into. I imagine a locker is not hard to break into. Small locks can be picked in a second or two by people with practice, which does not look different than retrieving your own phone.
The lockers are just so you have a place to put your phone. They are not secure in any way. Using a keyed locker just ensures you don't pick up someone else's phone by accident after the meeting. Remember that secure rooms live inside secure buildings, usually inside a secure facility with a fence and guy standing at the gate. And the guy has a gun.