There is more than one iOS app out there for use with Mobile Passport Control. The first iOS app for MPC is the free one that is from Airside Mobile, has been around for years -- as in from the start of MPC -- and is still available for free download and free use for years now. That original MPC app continues to be available from Airside Mobile and they added KCI about 5 months ago and HNL and OAK about 6 months ago.
The other app being mentioned in this thread is a free MPC app from CBP itself -- an app that is from 2021 and only works for IAD when CBP doesn't turn off MPC at IAD.
IIRC there have been at least 3 suppliers of iOS apps for MPC use, with CBP being the most recent provider of an iOS app for MPC use.