I suspect they don't care all that much. Its just laziness on garmin's part. They're selling millions of watches and can't modify their app to work better on an ipad.
Well, since the iPhone version is severely bugged (see Apple Health integration) and given that you can open Connect on Safari, probably they don’t see the ipad version as an effort to go through. It’s clear their iOS development team is not the best out there.
It would seem that the app is only built for one resolution, if you run it on an ipad, it appears quite small and you have to use the zoom feature.
You will also not that the keyboard when brought when in the app is the old style apple keyboard.
This would indicate that they have not yet re-written the app to use the later API's with reference to screen resolution/scaling and keyboards. If this is the case one has to ask what happens if Apple remove those old functions?