IIRC, iPhones (and every other smartphone) records variable frame rate videos (VFR), whereas professional cameras can record constant frame rate videos (CFR).

I think that VFR videos need to be re-encoded into CFR videos in order to be able to work with all the footage shot by different devices. It sounds to me like with the Genlock feature, it could actually be possible to record CFR videos on an iPhone and also synchronize the iPhone with other devices so that both video and audio does not drift relative to other devices. But that's just my speculation as I couldn't find any details about the Genlock feature.

I would love to know how the team working on 28 years later handled synchronization of the multiple iPhones they were using to shoot some scenes of the movie and if they got a helping hand from Apple, which perhaps allowed them to use some internal APIs to access hidden features of the camera stack...

One of the big use cases for Genlock these days is when you're doing virtual production with LED walls; you want to make sure the screen refresh of the wall is locked to the shutter of the camera. It's almost like 'vsync' in video game video settings, without it you risk seeing tearing in the backdrops.