It does not say "screw falling out completely would have been ok." The screw provided a single point of electrical contact for grounding. When it became loose (

It does not say "screw falling out completely would have been ok." The screw provided a single point of electrical contact for grounding. When it became loose (thus less surface area contact with the chassis and hence increased resistance in that part of the circuit) or fell out completely (zero surface area contact with chassis and hence infinite resistance in that part of the circuit) the current took a completely different path to the ground with catastrophic results.