I can believe this. But what if, instead of falling against the switch, the copilot, recognizing that he was about to pass out (e.g. recognizing symptoms of an impending stroke), intentionally attempted to move the switch to the "unlocked" postion (to make it easier for the captain to get into the cockpit quickly)? Due to a combination of confusion, physical incapacitation, and infamiliarity with a probably rarely-used control, he could conceivably have turned the switch to the wrong position even while he was attempting to do what he thought would be the best possible action.
The switch is designed such that the middle ("norm") position is the only one that's stable and will be retained without the user pushing the switch. I.e. the switch will always move back to "normal" when not actively pushed to either "lock" or "unlock". And with the switch in stable position, the door can always be unlocked from the outside -- with a short delay that gives the person inside the cockpit time to actively suppress the unlock using the switch. If the person in the cockpit does nothing, the door unlocks. So without deliberate and repeated activity from the person inside the cockpit, there is no scenario that would indefinitely prevent people outside the cockpit from entering.