Ask HN: How does Robinhood conduct a large-scale closed iOS beta?

From my understanding, Robinhood is currently in private beta of at least a few thousand users. For this beta they have an iOS app that is not available on the App Store. A normal iOS account only allows 100 devices to be provisioned, and TestFlight is limited to 1000 devices. The only way I can think that Robinhood is getting around this is through enterprise app signing, but this is intended for internal apps. Does anyone know how Robinhood (or any other company that has done this) conducts large-scale private iOS app distribution?