Apparently Fi doesn't warrant a dedicated carrier bundle, meaning Fi subscribers will need to wait until the generic T-Mobile MVNO setting gets flipped on. I've read (on Reddit) that Apple does this because Fi doesn't sell iPhones, but I obviously don't know if that's true. Afaik it took Apple 2 years and a half to enable 5G on Fi, which makes me believe it's indeed Apple being petty, and not some conspiracy theory.