1.5 trillion solar masses seems like an insane number for a galaxy that's over the hill on star formation, has about 100 billion stars, over half of which are red dwarfs.
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.)