My iPhone X definitely gets uncomfortably hot after less than 30 minutes of playing PUBG, and I don't even use a case. To illustrate my point, there are "gaming cases" with fans on them, so it's enough of an issue for companies to market solutions at it.
Ok, let's assume the iPhone's cooling system -- i.e. the metal back of the phone -- isn't adequate for long-running heavy loads. I don't think there's a deeper point behind this. Fit a proper passive heatsink and you're all set.
The Raspberry Pi would have been a better example.
