T-Mobile provided the following statement:
This is a fair usage policy, not a data cap. We don’t have a data cap. Customers still get unlimited data even if they use more than 1.2TB in a month (over 2X the average user – less than 10% of our customers). They’ll just be prioritized after other home internet customers for the reminder of the bill cycle, and they may notice slower speeds compared to other home internet customers in times of congestion. This ensures that all of our customers get a great network experience. At the start of the next month, those customers reset to normal prioritization until the next time they hit 1.2TB.
– T-Mobile
We’ve edited our post to clarify this news.