"produces sounds that exceed 140 decibels" is a nonsense statement. They probably meant dBSPL.
Colloquially, "decibel" is a widely known scale ranging from "I can barely hear that" to "holy crap, that's loud". 140 decibels communicates this fish is capable of a surprisingly loud sound - which is surprising. I don't really care if it's entirely accurate. I simply know that it's "surprisingly loud".
Sound pressure level measurements relate to the pressure at a point in space, and do not relate -- by themselves -- to the total sound energy being produced by a thing.
I can make an SPL measurement, in air, that is ~6dB higher or lower by halving or doubling (respectively) the distance betwixt the thing making the sound and the thing measuring the sound.
If a sound measures 100dB at a distance of 2 meters, then the same sound also measures 106dB at a distance of 1 meter, or 112dB at 0.5 meters, or 118dB at 0.25 meters.
A difference of 18dB is a difference of nearly two orders of magnitude.
The study says this fish made a sound at a level of 140dB at a measurement distance of one fish length of 12mm, in water.
To compare a 140dB fish-noise that to a the SPL of a gunshot, we'd first need to measure a gunshot -- in water -- at a distance of 12mm (or extrapolate for a larger distance).
And I have certainly not done this measurement myself, but I strongly suspect that the intensity of the gunshot will blow the noise of this fish out of the water in an an apples-to-apples comparison.