A workaround for OAuth-type flows not mentioned in the article -- or for any flow that requires going out to a web browser -- would be for Apple to cleanly establish a Handoff/Continuity mechanism from the Apple TV running tvOS to your iPhone, iPad or Mac. Which is to say -- using today's technology, an Apple TV app communicates with a server to see if you're also signed in on an iOS or Mac device with the same app installed. The remote service can then, on the request of the Apple TV, trigger a push notification on your phone, tablet or computer. You can see this happening already using OATH one-time passwords AKA two factor authentication, particularly in Facebook app's implementation, which presents a push notification on your phone when you're trying to do something on your computer.

So, that's one way they could easily overcome these limitations on the TV. See also: Existing Apple TV's approach to sign in, requiring users to use a computer or web browser to visit a page protected by OAuth and then type in a code from their TV to pass the session on to the TV. Or YouTube's even other approach of YouTube TV involving, IIRC, typing codes into your console/TV box from your YouTube website.