You can pin your certificate in your app bundle such that your app only allows certificates you specify or ones signed by CA's you specify. That clearly isn't the case here. Normal iOS operation will verify the certificate chains up to a trusted root certificate and it is indeed possible to load your own trusted root CAs on to a device for purposes of MITM. Again, some apps may pin their own certificates, but that clearly wasn't happening in this example.

I deal with this virtually every day.