Disney really wants to minimize the amount of people who sign up for their accounts in the app. Apple takes a 30% (ish) percent cut of the account fees when you sign up inside the app. I'm assuming Disney won't allow you to sign up inside the app to avoid paying the fee.
If so, releasing the app now would be pointless because the app could literally do nothing.
I’m assuming that’s only for people signing up with their Apple account. I can’t imagine that Netflix and other companies are handing over that fee.
Edit: Had a quick look at the Netflix app and you need to sign up using a browser.
It’s something that Disney could still do as downloading the App isn’t going to be an issue on launch day.