I'm not an expert, I just had to learn the basics recently to get my fishing permit. But the explanation given was that its function is allowing the fish to float / hold their position and save energy. Carps even have two chambers since they are bottom feeders and they can tilt their bodies mouth down more easily this way.
Yes. What you are talking about is a fish keeping a fixed depth.
Having a swim bladder enables the fish to attain neutral buoyancy. That way the fish doesn't need to expand energy to keep itself from sinking or floating up.
What the article is talking about is position keeping against the current in flowing water.
Swim bladder is good for up-down position keeping, the article's phenomenon is good for forward-backward position keeping.
Interestingly there is a way to use a swim bladder like construct to propel one forward. Underwater gliders do this, and the process is very energy efficient.
The way it works is that the glider uses its variable buoyancy device (an artificial swim bladder) to set a negative buoyancy and starts sinking. The wings of the glider turn this downward motion into forward speed. At the target depth the glider expands some energy from its batteries to set a positive buoyancy and keeps the forward momentum as it is ascending. Because of this they only need to use energy at two points (at the top and the bottom) in their saw-tooth like swim profile, and they can travel thousands of kilometers on a single charge.
