I've always assumed that was an example of a way you can shield bash someone, not the only way. A heavy shield is listed as a one-handed martial melee weapon, and a one-handed martial melee weapon can indeed be used as an off-hand weapon. It can also be used in the primary hand or with both hands. The text doesn't rule out those uses, it just highlights a use.
Arguing that a shield can only be used as an off-hand weapon seems analogous to arguing that a scythe can only be used to make trip attacks as the description says that "a scythe can be used to make trip attacks".