The key feature of dark matter is that it is matter that does not interact with electromagnetism. Finding a few dozen galaxies where there is not significant dark matter does not take away from the strong evidence for dark matter in virtually every galaxy that we look at (nor from unrelated lines of evidence like patterns in the microwave background).

This finding points to rare alternative means of galaxy formation and evidence against dark matter being an absolute requirement for any galaxy formation at all. That is entirely different from evidence against dark matter itself. As I said above actually bolsters the case for it since it suggests that all of our other measurements are not simply biased or explained by modified theories of gravity.