Correct. They throttle video streams even for those services that have not opted in.
Oh, and for a service to opt in, you need to disable serving over HTTPS, and you have to allow T-mo to modify the video, etc.
So, it is entirely not neutral.
Zero rate content inevitably comes back to cost consumers more. Work out the game theory: Someone always pays for this, and since the consumers are the money source in this every time, they inevitably pay one way or another. Zero-rating simply provides an easy way to distort the market, which benefits only the carrier.
A more reasonable plan would be to throttle everyone after they use some amount of bytes, and ignore content type, etc. and not require the providers to use insecure protocols, modification, etc.