Yes, UA (as well as AA & DL) do use buses and other ground transportation in order to get people to their destination. It is important not to be passive in IRROPS. Most agents would presume that people want to get to their destination and if UA's first flight at 8:30 was sold out, the next one is not until 12:10. But,others would rather spend the night and fly when space becomes available or ask for a reroute on AA xDCA.

The bottom line is that it is unclear what the family asked for. If they simply accepted the bus vouchers (or it was a charter) UA's reroute was reasonable.