I really don't think they should be able to call a plan unlimited if it is 5GB. They should call it the you get up to 5GB plan
Anyway, I believe he is correct in that the plan is only for 5GB of data per month and then you will be paying overage. It would probably be best to call TMO cs and make sure before you have an outrageous bill to cover.
I actually just called T-Mobile and they told me my unlimited plan has a 10GB cap then it starts to slow down. No overage charge though so that's good.
I think unlimited with a Softcap isn't really unlimited, it's virtually unlimited unless you are a person that really uses alot of data and in that sense it's rare that people hit that soft cap. In Canada we cap it at 5GB at least on Rogers, and I think most other providers cap it at 5GB as well, unless you have the promo 6GB cap that Rogers had our during the iPhone launch.
Really you don't need unlimited in most cases, hell you need less than 1GB in most cases as it stands today. I rarely go over 1GB or rarely come close to it.