I’m at work, so I can’t watch the movie, but I can guess what it shows.
Think of a vacuum cleaner. It doesn’t really “suck” - it reduces the pressure near to what you want to vacuum, and the surrounding air pressure pushes stuff into it.
Now think of a box with a low pressure in it, even a vacuum. Attach that box to a hose, stick the hose on your arm, open the valve. The low pressure volume “sucks” on your arm, but really it’s the air pressure around you trying to push you up the hose!
If a old diving suit loses pressure in the hose, say due to pumping failure, then you’re effectively attached to a low pressure area (the surface) by a hose, and the water pressure around you is going to try and push you up the hose. Which isn’t good if we’re talking several atmospheres of pressure. It would be like a super vacuum cleaner trying to suck you into it.
A simple non-return valve on the helmet would help out a lot. Or even a floppy hose that collapses on itself when it loses pressure.