The exemption that Netflix enjoys is this: "Reader Apps may allow a user to access previously purchased content or content subscriptions (specifically: magazines, newspapers, books, audio, music, video, access to professional databases, VoIP, cloud storage, and approved services such as classroom management apps"
It seems pretty clear to me that "Hey" does not give the user access to a previously purchased content subscription in one of the designated categories. Their subscription is not for music, video, books or any of the designated items. And the service they offer is not on the approved list of "exempt services" -- like stuff for teachers.
They don't fall in the exemption; therefore Apple says they are subject to rule 3.1.1.
Someone prove me wrong.