It just says, check mixture rich --> start engine --> keep micture rich at all times until engine shut down. There are no mixture checks that need to be done.
Checking the mixture on preflight is paramount. Cruise performance figures are based on the mixture leaned as per the POH (and you may need to lean at takeoff already if your field elevation warrants it). Also one of the memory items in case of engine fire is mixture cutoff. Two very good reasons to make sure it works as advertised before you actually launch
