That does not square with my experience of needing to have a decent wi-fi connection to successfully install and update.
I've installed multiple updates on our Model S while parked deep in a garage that blocked connectivity of any kind. We've also installed two updates on our Model 3 which has never been on any WiFi network (sadly its parking spot in the condo garage is just around a corner where it can't see our network, so we've never been able to connect at all, but it still gets decent LTE coverage.)