It's legit. The developer of the app shows Jado Interactive, which is what I'm assuming AFF runs on, if you check this link: http://www.asianfanfics.com/page/rules
I had it.. undownlobde fit bc I ran out of space but its alright. Theres just no 'back' button, so you're stuck on the fic you choose or you have to go back to the main page. that takes some getting used to. idk about updating fics from it so idk
Yep, I'm actually writing this from the app. It's somewhat the same as the website, but I don't update my fanfics using the app, so I don't know how that looks like.