Hollowpoints can't expand while in the barrel of the gun. What causes hollowpoints to expand is something called "stagnation pressure." Stagnation pressure is the measure of the pressure of a flowing medium if that medium were to be suddenly stopped. For instance, if an airplane was flying at 200 mph, there is a spot on the tip of the nose where the fast-moving air (moving relative to the plane) comes to a complete stop (on either side of this point, the air is moving along the surface). At this point, the pressure is much higher than elsewhere on the plane - this pressure is the "stagnation pressure" which is actually more a measure of the energy of the flow expressed in pressure units.
The point of this is that a hollowpoint expands when the stagnation pressure inside the cavity is higher than the pressure outside the cavity - the pressure "explodes" the tip outwards. When the bullet is in the barrel, the entire column of water in the barrel is moving with the bullet - there is no pressure difference between the water in the cavity and the water just outside the cavity (but still in the barrel). Therefore, the JHP will not expand until it leaves the barrel, and passes through relatively still water.
And as to the relevance of the test - I agree that it is kind of a "worst case scenario" in which the slide is retarded, an overpressure round is fired, etc. It can be used to gauge the "robustness" of the design - robustness meaning the ability of the device to function correctly in a wide range of conditions.
