That's evidence, not proof. We still don't have enough evidence to say conclusively that Dark matter is as we currently imagine it. And of course the same can be said about MOND.

Here's Milgrom's interpretation of the bullet cluster: http://www.astro.umd.edu/~ssm/mond/moti_bullet.html

I am not saying Dark Matter is something that actually exists that cannot be ruled out by other explanations (a revision of GR would be ideal IMO), just that this phenomenon makes the approach much trickier.