Already did as of earlier today. As a matter of fact it the update came down before iOS 8 was released. There page on iTunes says 4.2.0 "Sign In to Amazon Using Touch ID."
(this might be my first post after lurking for years, ha!)
You still log in to the app with your normal email/password. When you go to make a purchase, it'll prompt you with the TouchID popup. I think the update comments were a little misleading.