Feel free to ignore this...it is only food for thought.
Vacuum's tricky. If we watch some of the educational shows like "How do they do it?" and "Factory made" etc, we can see that they use vacuum to make components conform to a mold. The atm pressure is making it conform, and the pressure is being exerted across the plastic.
My theory: the plastic mashed the meat, and some air comes out, but mostly liquids get displaced, only to be replaced with different fluids each time the meat re-expands.
Like many of you, I milked cows for years, and vacuum got to be one of my best pals. (Since I cannot possibly milk a modern cow by hand.) The milk is expressed by a differential pressure across the teat...the udder being under atmospheric pressure (plus ejection pressure, which wanes,) but the milkers themselves are under vacuum. Pulsators keep the blood from pooling by releasing the vacuum, producing a massaging action in the liner. I think the meat is likewise being massaged during the marinade.