"I wear a black skirt."
I agree, but I suppose the devs' idea is that once the kanji is introduced, you will remember the sound from the context and earlier practice. I am also studying the kanji through Heisig, and I suppose it depends what approach you take. I'm honestly not sure which approach is better.
Im studying the Kanji in a Book and it definitely helps to learn the hirigana, and katakana here.
Most Kanji have two or more common endings. But also have less common, and irregular sound endings as well.
So Kanji compounds, can be hard as heck to decipher. In this app we are essentially learning how to "spell" correctly. And when to use certain pronunciation.
My phone isn't allowing me to type the Kanji, but a quick example would be the character for 5, it can be pronounced GO, as well as itsu. Here in the app you will learn when one is used in preparation for seeing th kanji. Essentially learning context and structure.
Long winded, but hope it gives some light as to what I feel Duo is trying to achieve.