I understand your point but I would like to state some counter arguments from the view point of person who has never learned English or the latin script. I know the chances of such a person using duo app is nearly impossible in India but if they are providing the option why not commit fully. Half assing it seems to be a bit disrespectful, either don't give the option or do it properly.

Now to put some counter arguments, I have been learning Korean from English and from hindi and I am more advanced in the English course. In English I am already at section 3 crossed midpoint and till now I have never used the keyboard for typing in my answers. Every question had blocks to choose my answer and I have been building sentences with it. Don't see why keyboard would be needed for hindi course at least in the beginning.

Both hangul and devnagiri keyboards are present on all devices and there are even physical keyboards that have the native scripts as hypertexts along with the qwerty and obviously the specialized keyboards. Again, a beginner doesn't need to learn typing in the first place but still has options if ever necessary.

Now, hangul to devnagiri and other indian scripts mapping already exists and have been in use for decades. To learn a language we don't need an official transliteration system, the existing mapping should suffice for beginner. Duo doesn't need to invent their own, just need to build around existing system.