I agree with you, but the folks who made this app clearly don't.
The fact that these devs put so much time and energy into making something that I cannot imagine wanting to use for the reasons you describe makes me think that I have a very different relationship to my mobile devices than some other people do. Another example in the same vein is typing emails or docs. I don't ever type anything longer than a couple of sentences unless it's extremely urgent. But there are many people who don't seem to mind one bit!
I think the developers use Asus Transformers (tablets with associated keyboard docking stations that turn them into de facto laptops) extensively, so having a full IDE that can run on one of those would indeed be a boon -- even if most Android users have little use for it.
But why?? The point is that I would never choose to do development on a smallish device. In fact, I would always choose the largest, most powerful device available! Multiple screens, speakers, lots of ports, power, etc.
Maybe there is a market for commuters/travelers? Even then, I would prefer a large-screen laptop.
Regarding typing, I think a lot depends on the keyboard you are using. With default keyboard, yeah there's no way I'm typing more than a couple of sentences, as you said.
But with a better keyboard (e.g. Swift on Android), I can easily type a few paragraphs. Because text prediction and spelling correction is so damn good, I just need to press the keys roughly near the one I intended (e.g. jekko -> hello).
Although, I could type natural languages, there's no way I'd be typing code. Simply the number of special characters required makes it very slow and frustrating. And if you need to be mobile, you can just buy Chromebook (or similar device), install Ubuntu/Arch/whatever and happy coding.
The fact that these devs put so much time and energy into making something that I cannot imagine wanting to use for the reasons you describe makes me think that I have a very different relationship to my mobile devices than some other people do. Another example in the same vein is typing emails or docs. I don't ever type anything longer than a couple of sentences unless it's extremely urgent. But there are many people who don't seem to mind one bit!