...they throttle at the phone-number/SIM. Even with a VPN your phone is still auth'ing itself to the cell towers, and those towers know what device is sending which traffic.

What this prevents is allowing say Youtube to pay TMobile to never throttle their traffic.

VPNs work at a higher level. They have to see the radio traffic to be able to deliver packets to your phone, which is where billing and access control happens (this is why you can’t spoof someone else’s IP to avoid paying your bill), and at the IP level your VPN traffic is carried from your carrier-issued IP address to your VPN provider’s addresses.

The one legitimate argument here is that this prevents traffic shaping based on the destination, which T-Mobile uses to do things like offer unlimited streaming separate from your general data quota.