First off, autostart does not close the spar valve, it closes the engine (hydromechanical unit) fuel valve. Second, one of the potential reasons that autostart has failed is because the EEC can't properly control the fuel valve, which means it may not have been able to close it. Putting the fuel switch to Cutoff closes the engine fuel valve via an independent circuit. It also closes the spar valve (which autostart can't control).
If autostart already did the 30 second dry motor, there really isn't any reason to repeat it. However there are failure modes where the autostart system won't (or isn't able to) do the 30 second dry motor. Hence the instruction to dry motor.
BTW, on the GE90, autostart is resident in the EEC, so it's not really independent.