Correct. You can tether with the iPhone on tmobile even if you don't have tethering added to your plan. It's probably because the iPhone isn't an officially supported device in their system.
Currently this even works if you're on the truly unlimited plan so those in refarmed markets can tether as much as they want. I suspect Tmobile will close this loophole as more areas become refarmed.
No those are throttled after 5GB. The Truly Unlimited Plan is only available if you convert to one of the current Value or Classic Plans that T-Mo now offers.