How do we know that comets definitely mase and not just fluoresce?
I'm not sure if this is what you're asking but:
fluorescence:   excited electrons randomly emit photons.
Superfluorescence:  excited electrons emit photons, and there is some self-stimulation, leading to a strong short pulse but little coherence.
Superradiance:    strong, shorter pulse,  much self-stimulation , high coherence in output.
Lasing/Masing:  almost always requires a cavity of some sort, leading to directional coherent output.
I cannot tell exactly what the authors of the article consider qualifying as Maser vs. Superradiance.  possibly the  material volume in question (on the comet) is elliptical enough to lead to a more-or-less directed beam.