So reading through pages and pages of comments here, other websites, I think I finally figured it out.
For new customers... It sounds like you can either do the interest-free EIP plan OR have the free 200mb per month. You can't have both.
The only way to get both is if:
You are already a T-mobile postpaid phone customer and are adding a tablet to your account.
If you are a new customer, you must pay for the tablet in full (alternatively, you can bring your own tablet to tmobile). Having the tablet paid in full allows for the 200mb plan.
Sources below: Taken from the tmobile website
Buy from us with interest-free financing.
We make it easy to get the tablet you want with our EIP interest free financing, with approved credit. EIP is exclusively available to T-Mobile customers on monthly Simple Choice plans (plans start at $0/month if you also have a voice plan with us, $20/month otherwise).
Also, they list 4 ways to get the 200mb per month free. Note that none of these ways would allow a new customer to t-mobile do the EIP plan and also the 200mb free per month.
Four ways to enjoy free data for life.
1. Bring your own tablet. Use a tablet you already own and love on T-Mobiles nationwide 4G LTE network. Simply activate a Mobile Internet account with a SIM card.
2. Add a tablet to your T-Mobile account. If you already have a postpaid phone line with T-Mobile, add a tablet to your account and enjoy low monthly payments and 0% interest with our exclusive Equipment Installment Plan (EIP).
3. Switch over to T-Mobile. Let one of our award-winning retail stores introduce you to our selection of smartphones and tablets, and enjoy the benefits of EIP when you add a tablet to your phone purchase.
4. Just buying a tablet. No billing relationship required. Shop our selection of great tablets. They come with a T-Mobile SIM to un-leash our nationwide 4G network whenever you need it.
I think I finally figured it out, I am just disappointed. I am not a phone customer - yet. I am still on contract for another year.
tl;dr
Current tmobile customers adding a tablet to their account can get the EIP loan AND get 200mb free per month.
New customers can't have both.
NOTE: Who knows what tomorrow will bring though, since the message has been inconsistent.
Except that I'm a new customer on EIP w/$10 On-Demand plan (200MB only). Not to mention, it doesn't matter what level you're at, you get the 200MB added to whatever allowance you purchase otherwise.