Freezing? That's not really true, or at least, not an accurate description.
The altitude is low, about 1,500 feet. I think it is unlikely it makes any difference to the physical uncharacteristic of the battery, more likely it has something to to with the cell towers.
Cell towers broadcast line-of-sight. If you are up in the air, you are exposed to a lot more cell towers, since the line of sight is so much further. I am speculating that this increase in a wider coverage may be confusing the phones' operating system and shutting it down. Just a guess.
It is not a cell tower problem from being up in the air. I have flown in private jets, with my phone on for 3-4 hours at a time. The impact on the battery is minimal, no different really, than being in an area without service.
I agree, that freezing is not a good term, it's more of being very cold and as you know, a cold battery either works poorly or not at all.