I found the catch. After calling T-Mobile again, this time I was told that the offer is only available for customers with an unlimited data plan. So either you have to have a T-Mobile ONE plan, or a grandfathered Simple Choice plan with unlimited data. So since my Simple Choice plan only has 4GB of data on each line, I don't qualify. Upgrading my plan to unlimited data would cost another $82 per month for 8 lines. So not worth it.
And there is my bright idea out the window from a previous post. I'm grandfathered as well.