The vast majority of the mass in most galaxies is dark matter anyway, so the number of stars is not so important. Remember, normal matter is only about 5% of all the stuff in the universe.
Given that they just revised the mass of the Milky Way by a factor of five, we should not regard all the other numbers as being absolutely certain. They may be the best numbers we have now, but they are subject to revision, sometimes drastically.
There's already a lot of variation in the proportion of dark matter to visible matter in different galaxies. Revising one galaxy doesn't change the statistics that much.
(However you are right that from a Bayesian point of view, the Milky Way is the galaxy we have the most data on. So should count for a lot more than a random galaxy.)
Yes, that’s the basically the reason this paper gives for the large discrepancy between their estimate of the mass and prior estimates. The Gaia satellite measured the positions and velocities of billions of stars in the Milky Way, information we’ve never had before, so a new estimate being significantly different from the prior estimate is not _that_ surprising.
But the 5% number that I quoted above should not be taken as an extremely precise number. You can plainly see that it has only one significant figure; if the real number turns out to be 4% or 6% then I don’t think cosmologists will be too surprised.
