It looks like their app requires iOS 13 which requires an iPhone 6s or newer (though it can run iOS 14 and will run iOS 15). The iPhone 6 won't run iOS 13, it had five years between its release and the release of an OS it can't run, that still seems like a respectable amount for a phone.
Apple is way better at supporting the longevity of their devices, through build quality and continued OS support, than the Android world is.
Well, the idea with iOS/macOS is "Have a recent OS+Device, we change things all the time and demand that everyone moves along in 5-6 years or so".
With Windows it's "We support 20+ years old programs just fine. If you're OK with having to deal with 8+ layers of GUI code and legacy cruft, we're your thing".
It's not like this hasn't been the case for decades, for it to be a suprise...