As bumbo stated. You will be able to watch on any device, Mac iPhone iPad or Apple TV so long as you are signed in with the same Apple ID as the device you activated your subscription on.Thanks. Just so Im clear.
I bought an iphone 11, I sign into my phone with my apple id... install the app on my phone, then open the app and activate the sub on my phone.
But how then do I watch it on my ATV4K?
That’s fake news. Did you read the rest of the paragraph you quoted from the terms and conditions?From the terms and conditions https://www.apple.com/promo/pdf/EN_US_ATV+_Promo_TandCs.pdf
"Offer must be claimed in the Apple TV app within 3 months after first setting up your new device."
The key words are within 3 months of setting up your new device. If you got the new iPhones on launch day in late September, you already have lost the first 40 to 45 days of that 3 month period if you set up the phone when you got it. So you really only have until mid December at the latest to sign up for a year of free ATV+