The difference is that the Bullet cluster is an observation that needs to be explained for you to have a viable model of the cosmos. Any model which cannot explain it is not viable.

Dark matter models can explain it ... and also can explain all of the other observational data. Dark matter models are viable.

There is no known model of modified gravity that is viable. Every one that has been proposed to date is either too vague to make novel testable predictions (in which case you can't really call it a model to begin with), or it has made novel testable predictions that were found to be in conflict with observations, leading to the proposed model being falsified.