I don't know, I'm not an iOS user but it would hugely inconvenience me if the only way I could use Dvorak on Android is to use a keyboard with a hardware Dvorak layout.
Edit: Indeed, it appears that iOS supports Dvorak layout for external keyboards already; this is supporting the native on-screen keyboard
This is a little ambiguous, I assume you mean a (rare) hardware Dvorak keyboard, not that iOS supports remapping a standard ANSI external keyboard to Dvorak - but correct me if that's wrong.
Your _muscle_ memory makes you good at moving your fingers (or thumbs) to an absolute position on your keyboard (or screen) - but to do that you must first decide what absolute position your finger (or thumb) should move to (where is the J key?).
Qwerty is useful for "swipe typing" on a touch screen and Dvorak/Colemak is great for touch typing on hardware and the way "muscle memory" works those are such different media/muscle movements that they have separate "muscle memory".
It faced the same problem as all new keyboard designs, getting people to use it.
If you are on Android, you can find plenty of research project keyboards that are indeed more efficient to use, after you get over the learning curve.