Why people use self-custodial wallets on iPhone is beyond me. You are totally at the mercy of the wallet provider as there is no way to independently verify binaries.

By my estimate, a wallet provider would not be able to target only non-jailbroken iPhones but he can target only non-jailbreakable phones which are most of the phones in use and certainly most of the phones storing significant amounts of Bitcoin as that's all iPhones released in recent years.

So even if somebody obtained a binary from a jail-broken iPhone and reverse engineered it or reproduced it from public source code to find potential backdoors, this effort would be irrelevant for users of the latest iPhone for example as the wallet provider can deliver a different - backdoored - experience to those.