T-Mobile just started advertising that they can be used on any flight. I think this is just a second signal that the market is moving towards Free(ish) WiFi on planes, boats, etc.
Plan 1 won't encroach on plan 2 yet, unless all you care about is Whatsapp.
Regardless if you are a T-Mobile customer using that deal or not at all, you join the in-plane WiFi AP, it goes to the captive portal, and there you get options like "tmobile customers offer", "single flight pass", and "monthly flight pass". Once you pick your option and get through it, you have internet access. And regardless which option you went with, you get the exact same type of internet connection as everyone else on that flight.
From what I've personally seen, it was done through GoGo Inflight Internet[0] heavy majority of the time.