Well, there are several mechanisms that might result in embolism. One is laryngospasm, which is not unlikely if somebody has aspirated water. Another is blockage of the posterior pharynx by the tongue. Another is obstruction of the airways with aspirated water. Remember that all you need for embolism is that the gas in the lungs is expanding faster than it can escape, not that it has to be blocked altogether.
Unconscious or dead people don't hold their breath. Undertakers don't have to deflate their "client's" lungs.
Don't confuse an obstructed airway preventing inhalation with the ability to hold your breath.
In first aid rescue breathing you extend the airway to:
(1) Allow the casualty to breath for themselves or
(2) Allow you to apply rescue breaths.
You will never encounter a situation where, when you open a non-breathing casualty's airway, they exhale.
Extending the airway underwater simply wastes time, overrides the victim's gag-reflex, allowing water to enter the lungs, possibly worsening their condition if they are succesfully resuscitated and gives you something else to worry about during a rescue when you've already got your hands full.
These staements are contridictory. It would be interesting to see a study on this subject. Unfortunately, I think that all of the training on surfacing an underwater casulty is based on "best quess" medical theory.
I tend to side with Hickdive because I can not see how an unconscious and relaxed person is going to hold in expanding gas in their lungs.