Maybe not in the Android case, but in the iPhone case, it is pretty much true and there are clear ways to deal with the variances in capabilities between iPhone models. It's a much more predictable platform than for PCs or even Macs.
It isn't true in the iphone case either, they don't create a new appstore when they release new hardware generations so they don't check if apps works fine on older hardware nor do they guarantee that the software you buy in their appstore continues to work after updates. Instead it is up to the software provider to update things when Apple breaks their product and to inform the user of which phone can run the app.
