Lutron uses their own proprietary network protocol, they are not actually part of your smartthings network. There is a cloud to cloud integration instead. Consequently theIr plug-in dimmer only repeats the Lutron messages, none of your other smartthings devices.
The smartthings hub is a single box that contains a Z wave radio and a zigbee radio. So normally when people are talking about repeaters, they are talking about a repeater for one of those two protocols.
See the following FAQ, and in particular read post 11, then go up to the top of the thread and read from the beginning.