It's not that the dark matter distribution is isotropic (it's not, as you note). Think of it like this: locally for a given object in the universe, we can work out the amount of dark matter necessary to describe one of its anomalous properties under the DM model. That same amount will work to explain the other anomalous properties locally for that object. You can do this at any point in the observable universe, including the unusual cases like the DM-deficient galaxies, and the model still holds up. MOND and related theories struggle in this regard, which is why DM is viewed as a more likely explanation (whatever the underlying nature of DM might be).