Not true... the very first time you enable DCS support, it adds its entry to your export.lua and installs its export script. DCS may require a full restart after the first install (though I don't think it does) but at a minimum, you will need to restart the flight. If a new version of TelemFFB includes an update to the export script, you will need to restart your flight if active, but do not need to restart DCS.
You can start TelemFFB whenever you wish. Before or after DCS start, or even while already in flight.