I feel like you think this means munk-a is merely refusing to update past iOS 10, but there are devices that simply can't. (The ones stuck specifically on iOS 10 forever are the iPhone 5 and 5c.)
I think the original message was poorly worded, I think they were imagining that devices that can't run iOS 15 would essentially be bricks because every app maker would build their apps in this way that only works on iOS 15 devices. Of course, this will not happen. It has long been the case that developers need to balance the advantages of using techniques enabled in newer OS versions against the advantage of supporting a wider range of OS versions.
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...
The iPhone 5/5c was released nine years ago. I'm just not crying for it. If your app dev isn't updating for iOS 15, you're stuck with the old version anyway. Sorry your decade-old device can't use the latest technology.