From above:
No-mount means - you are in such a tight space that you won't fit with your air tank on your back. So you need to remove the tank, and feed it through in front of you by hand. While maintaining air flow.No-mount restrictions are no joke, but they're not impassable. We do it all the time in cave diving, and while I'm never super comfortable with them, I'm also not peeing myself at the thought.
We train in this in firefighting. Some spaces are too small to traverse with all the gear on, so you need to become smaller. Our routine testing involves going through some culverts, and some collapsed building simulators, where we need to do this. In total blackout conditions.
It's freaking terrifying, especially since if you manage to pull off your facepiece/mask in this situation, you probably die with the next breath in real life, if it's a firefighting situation. It requires a lot of drill and developing confidence in your gear and your skills to be able to do this consistently.
Good gracious, those poor kids.