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According to a thread on Reddit Single-Sign-On is active on some apps - has pics as proof:

https://www.reddit.com/r/appletv/comments/5338jv/single_sign_on_appears_to_be_working_for_a_few/

oI9Voyob7m6n57F7D5UjECludSjAHwd7T-Enw4T8tcY.jpg
 
Dark Mode is a welcome update although I wish it had a way to set the background to 18% grey.
 
So... the new Photos has "Memories" but not "People". My kids like the People automatic videos.

I guess they're not doing it because the people you recognise on the IPhone aren't synced through iCloud... so you'd have to do all the AppleTV people separately to the iPhone (and Mac). Can't think of another reason not to.
 
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