The galaxy “without dark matter”, NGC1052-DF2 is not an evidence against MOND.
Let me explain:
To weigh a galaxy, van Dokkum and collaborators measured the speed of bright globular-cluster-like-objects in said galaxy. The more massive the galaxy, the faster these globular clusters move. They measured the average speed of the globular clusters to be ~10.5 km/s at 90% confidence.
Of course, the amount of mass in the galaxy, and thus the speed of these globular clusters must be somewhat dependent on how bright the galaxy is. This is because stars and gas are bright and massive. The brighter the galaxy, the faster these globular clusters move.
However, in dark matter theories, I can arbitrary increase the velocities of these globular clusters by adding dark matter into the galaxy. This is because dark matter increases the mass of the galaxy without changing the amount of light emitted by the galaxy.
The claim by van Dokkum et al. is that the velocities of NGC1052-DF2’s globular clusters are so low that all of it can be explained by the bright components (stars and gas) of the galaxy. In particular, van Dokkum et al. calculated that MOND will give an average velocity of ~20 km/s, way beyond the “10.5 km/s at 90% confidence” they measured.
Their MOND calculation is wrong because in their calculation they assumed that NGC1052-DF2 is an isolated galaxy. In reality, NGC1052-DF2 is a satellite of the giant elliptical galaxy NGC1052 (as clued in by its name). Correcting the MOND prediction for the existence of the giant elliptical galaxy, the average velocity of the globular clusters predicted by MOND becomes ~13.4 km/s, which is within the error bars of van Dokkum et al.'s measurement.
A different team of astronomers has re-weighted NGC1052-DF2 and found that it is inconsistent with the lack of dark matter claim of van Dokkum et al.