Yep! But that mmWave spectrum has been deployable for quite a while now - last years iPhone supported n260/n261, and so did the Galaxy series as far back as the S10. The only phone that T-Mobile currently sells that supports n258 is the OnePlus 9 Pro 5G.
Having a mainstream device such as the iPhone supporting n258 will greatly increase T-Mobile's amount of deployable mmWave. Tampa, for example, would go from 600MHz of deployable spectrum to 1000MHz (400MHz n260 + 200MHz n261 + 400 n258). NYC, which is currently only operating with 100MHz of n261, will go from 500MHz of deployable spectrum to 900MHz (400MHz n260 + 100MHz n261 + 400MHz n258). Hopefully, having more deployable spectrum will push them to expand deployment.