I don't know how you could tell whether the glottis stops the throat completely or only partially without sticking a video camera down someone's throat. Do you suppose that's ever been done?

By the way, I think the glottis is what cats use to purr. I understand no one knows for certain, because the minute you stick anything down a cat's throat to get a peek, it stops purring! I think you'd have a similar problem trying to observe a glottal stop.

Maybe an ultrasound image would work.

My hunch is, all glottal stops close the throat completely, and the only difference between a soft-subtle glottal stop and a loud-distinct one is the amount of air pressure you put behind it -- the same as any consonant, like B or P.