Non-removable just means it's soldered down. Open a phone and you will see that the batteries certainly have just the kind of plastic shell that they always had.
most phones and tablets i've worked on recently certainly do not have any plastic shell on non-removable batteries; unless you consider shrink tubing a shell.
They are usually constructed of layers of battery material, wrapped in kapton tape or an equivalent, with the entire package shrink-tubed and a pigtail coming out the side for the device interface.
One could easily crush the unit by hand, the construction offers little to no protection from physical forces.
