AT&T is playing marketing games. Don't believe the hype.
More relevant and important:
I'm no expert, but in talks with our enterprise network vendors, they confirm there are potentially serious limitations with mmWave. I was told it's probably best for outdoor use in small areas. Who knows. Anyway, this quote on the wikipedia page sounds right to me:
"Millimeter waves propagate solely by
line-of-sight paths. They are not reflected by the
ionosphere nor do they travel along the Earth as
ground waves as lower frequency radio waves do.
[1] At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage.
[1][2][3]"
https://en.wikipedia.org/wiki/Extremely_high_frequency
Before signing a big contract with Sprint using mmWave for 5G, I strongly encourage you to find a way to test it, especially within buildings, and especially concrete buildings.
I'd love to be wrong and welcome clarifications and corrections.