I'm afraid i'm not wrong. (Don't try to teach an old man to suck eggs)
Well... you could first set the lever, then push the release, then insert a cable relea... uhmm... too late.
I think i didn't make it as clear as it should be.
The lever locks the shutter release button. Use it when you release by pressing the button with your finger, and the lever will keep the button depressed (and thus the rear shutter open) until you reset it.
But use a cable release, and the tip of that will not move the shutter release button, but push the rod the button too would push if it were used.
Let go of the cable release, and you'll hear the baffle doors slam shut, since there was only the pressure applied by the cable release to keep them open.
The locking lever than does absolutely nothing at all. It only locks the actual - then not used - shutter release button.
Since you must use a cable release for long exposures, the locking lever had no real use. It will not be missed.