Just because the act "giving your phone to a person" is the same, it doesn't mean it's the same thing.
Trust and incentives are a thing. Just like depositing money in your bank is different to telling a stranger on the street to keep your money safe even if the act of handing over valuables is the same. It's not in the interest of the bank to lose trust, just like the founders of that app are not in the business of cracking the phones of fellow founders.
It would absolutely be worth the investment to put 99% of attendees' phones in normal boring lockers, to spend an hour with _one phone_ belonging to a high-value target attending such an event.
No way to prove that happened here, but it's the sort of thing that any crook worth their salt would be spinning up a fake phone-locker business for. "We'll provide phone lockers for your event" would get you a lot of boring-but-profitable gigs, and the occasional chance of a lifetime.
You're not getting it - I'm putting forward that the act is the same regardless of the intention.
You're exposing the same level of vulnerability whether you give it to a border guard, the cops, or your nurse in hospital.
It's fundamentally the same action and same vulnerability, papering it over with "you can trust this particular system" means nothing - the act is the act.
Trust and incentives are a thing. Just like depositing money in your bank is different to telling a stranger on the street to keep your money safe even if the act of handing over valuables is the same. It's not in the interest of the bank to lose trust, just like the founders of that app are not in the business of cracking the phones of fellow founders.