The deal happens today but for everyone who had T-Mobile One prior to today, you won't get this deal, unless you upgrade to the tax inclusive plan.
This is mostly for new customers and if you already have T-Mobile One and Netflix, then getting this deal will actually cost you more.
I'm on the "old" T-Mobile One plan and I have two lines. So, this is how the deal would look:
New T-Mobile One Plan w/ Netflix: $120+ at least (depends on taxes)
Old T-Mobile One Plan, plus my own Netflix account: $109.99