Mike, your suspicion is wrong. I wonder why you did post without knowing the answer. And Miguel, that refers to the external battery charger. It's specified to 8,4v as that's two times (as the X series battery consists of two cells) the 4,2 LiPo cell voltage (fully charged). That has *nothing* to do with the USB charing function of the X-T2.
LiPo batteries require special charging, hence the X-T2 has a built in charging circuit for the internal battery. That one is fed via USB power supply lines. The initial post is talking about iPhone/iPad "chargers". Which is technically not correct. They're just power supplies that provide power via the USB socket. As Bobj already mentioned, the voltage and the pinout is standardised on USB. All of these USB "chargers" from devices like Android phones/tablets work fine with the X-T2. Same for cigarette plug USB power adapters. They're just linear converters which drop the 12V from the car battery to 5V and provide it via the USB plug. No need to worry about the battery. It also does not matter how many amps the power supply can deliver. If it is too few, charing will just take longer. if the power supply can provide more (e.g. 2A is quite standard on nowadays USB "chargers"), the internal charging circuit of the X-T2 will just draw the current it needs.
So long story short, yes, you can use the iPad/iPhone USB power adapter to charge the X-T2.