The answer referenced in this question doesn't address the plausibility of evolving lasers in the first place. For a laser to appear in the anatomy of a species in a fully-functional state where no such organ existed in the previous generation is a probability so small as to be an impossibility.
In order for a laser to evolve, its less-evolved precursor organs must be of use. Eyes evolved from mere light-sensitive pits to modern eyes in steps, each step just a bit more useful than the last. Unfortunately, I can't think what precursors to a laser might be useful enough to evolve.
However, my lack of imagination shouldn't be taken to mean that biological lasers are an impossibility...
Now... to speculate on the gas inside the laser... a chlorine/ iodine/ hydrogen peroxide/ potassium hydroxide laser sounds quite toxic and difficult to produce, and there are better, more easily obtained gases with which to fill a laser, such as carbon dioxide.
Carbon dioxide has the advantage that it is produced as a byproduct of cellular respiration, so it will be readily available, and could do just as good a job of ionizing air as any other laser filled with a more exotic gas mixture.
While Helium is a common gas used in CO2 lasers, it is not strictly necessary, and the gas mixture may be carbon dioxide and nitrogen, both of which would be readily available and easy to store.