If that is what HTC says then that's what it is. Not much we can do about it.
I've asked multiple places including head of T-Mobile and HTC on Twitter and can't get a straight answer either. From my own knowledge I know T-Mobile no longer has a listing inline, several stores do not have it, and I know one store near me that still has them in stock. It appears T-Mobile discontinued selling them once stock is depleted. My guess the stores that have them ordered a high initial quantity. T-Mobile swears they will still support it, I have yet to see them live up to their claims since I've seen several updates older than a month out and we haven't been updated.