As Bobby said, the extender would be instead of SmartThings. Both are Zigbee coordinators, and a sensor, for example,can only report to one.
The Harmony “hub” is the piece you need for Harmony to work with SmartThings. But if your remote is older, it’s likely that it needs a firmware upgrade from Logitech in order to work with the harmony hub.
I know it seems weird because we’re saying that the harmony hub works with the SmartThings hub where the harmony “hub extender” does not and it doesn’t seem like you would need two hubs. But you do. The two companies just use the word “Hub” differently.