This teaches intervals like Duolingo teaches language rules. You sort of pick them up because you need them to figure out the small melody it plays. But you don't get the concept of a 'fourth' or a 'fifth' and there's never a moment where the actual rules are explained.

That said, I think it's very useful for what it is and highlights that whatever your view on AI, there is a niche here that AI can fill that people otherwise would just not build either because they don't think it is interesting, or because no one would pay enough for it.

I addressed that. You should read a book to learn the definition of intervals. But in addition, there's no substitute for ear training. Grinding on interval identification is just as valid as this. Once you get to a level where you can identify intervals on the keyboard, the skills are pretty transferable. But there's just no way to learn what a fifth sounds like by reading a book. You need something like this. There is probably room to add a mode that says "this is a fifth" after you identify a fifth. Or to choose a named interval or chord quality based on hearing it. But I don't think any of that diminishes the utility of what's here.

FWIW I think it's probably more useful to play what you hear than it is to be able to name it. Although they're both good.

It's ear training, not theory training, right?