That's correct if you pay for the One Plus International - which includes unlimited high-speed tethering. Even at 28 gigs, they claim you are only throttled when connected via congested towers.
We are on their older Simple Choice Unlimited which was a better value. We also have a hotspot device that gives us 22GB at high-speed, then low speed after that.
The key with T-Mobile is if you are going to watch videos with it, run the app on the phone and use something like screen mirroring with a Roku to put it on your TV. Data on the phone itself is unlimited high-speed. If you turn off the "binge-on" feature, the video will be higher quality. On their new plans, you have to select high-quality video by logging into your account every day.