The total amount downloaded or uploaded really isn’t the right thing to care about. They just need to manage capacity on their network on a per cell basis and then wherever they aggregate to. I think if every user in a given region was completley saturating their 100Mbit/s link all at the same time, yes they would melt. Fortunatly as they onboard new users they can monitor capacity and daily activity and make sure they cap the number of users in an area before they have capacity issues. As usage patterns change over time (and WFH becomes more prevalent due to covid) they will also have to keep up with the demands for more bandwidth.

SpaceX is similarly doing this with Starlink providing a 100MBit/s+ home internet link.