I saw the original post. I was certain it was EMF and that the helium theory was nonsense.

But damn, it actually was the helium. This is one of the best comments of the thread:

In the original thread, the suggestion was MEMS resonators. The iphone 7 uses one of those instead of a quartz crystal for the 32kHz oscillator. Someone suggested it could be using that (which is time keeping accurate) to regulate the main clock. Get a little He in the package, and it might even just stop resonating altogether. no clock, no processing. you couldn't even wake the device from sleep or put it to sleep. It matches what is shown in the video very well.

Now, finding info about what resonators / oscillators iphones use has proven difficult. I've been looking for a phone to test this with, but finding a 7 or newer has been tough.

https://www.reddit.com/r/sysadmin/comments/9si6r9/postmortem_mri_disables_every_ios_device_in/e8pak41/

So there's a micro-mechanical-electrical device that acts as the system clock instead of the typical quartz oscillator. And it needs a near-vacuum to work, and the helium fits through the seals and gets in to the oscillator, which in turn breaks the whole phone until the helium eventually escapes. Because that particular oscillator isn't the main one for the CPU, but does govern the power supply.