It's not about peering.

In return for zero rating the video data for the video providers in the BingeOn program, they agree to serve video with a fixed highest bitrate, which saves on tower spectrum (which is far more important than whatever bandwidth consumption T-Mobile is experiencing at its peering points).

This makes sense. If you're a consumer who doesn't care about the bitrate being limited (assuming the highest bitrate is based on mobile device screen resolution), its not a problem. You get decent video quality on your device, T-Mobile doesn't have to work as hard at network management. If you want to opt out for whatever reason, there is a toggle switch to do so.

Its a clever solution to limited last-mile mobile connectivity.