The slipping down I could visualise as well as diesel 911 mentions. Since he had the spring removed to attempt the change of the valve seal. He would have noticed a tremendous amount slop if it where not present. Thats if he was using the piston to hold the valve up rather than air pressure.

Normally if the guide is found loose it is still in it's relative position. A loose guide is not really that uncommon with these heads. This would be the first instance of actaully slipping down I have ever heard of on site if that proves the case.