"We stress, however, that the canonical interpretation of AGNs causing the broad Hβ lines also remains viable."
What’s the layperson’s interpretation of this?
They have offered an interesting hypothesis (all those stars packed into a small space) but this other, perhaps more conventional, explanation for the observations is not ruled out.
They're trying to figure out how fast the galaxy is rotating by looking at the redshift from the stars. Some are moving towards us and others away. That spreads out the distinctive frequency of hydrogen emissions.
They think it's broader than the conventional explanation (active galactic nuclei) can explain. But it can't be ruled out.