The closest equivalent is probably the War on Terror, insofar as both purport to be a reaction to actual terrorist attacks and to be aimed at preventing future terrorism. We may then compare them in terms of (crudely) total actual suffering caused (less crudely, we need to take into account total potential suffering prevented - recall how Hiroshima was justified - but evaluating counterfactuals is a tricky matter so I will set that aside here). Even assuming the worst version of what's said to be happening in Xinjiang, there is no equivalence (in terms of actual suffering caused) with the War on Terror, which is estimated in 2009 to have resulted in 3.1 million direct or indirect deaths [1]. From a crude utilitarian point of view, there is simply no comparison: any clear-headed utilitarian given the choice to prevent exactly one of two will not feel that there is even a decision to be made.

Of course a natural reaction is to say that the Uyghurs have a legitimate cause, and the 911 attackers didn't. However, the official US theory of the motives behind the 911 attacks is "they hated us for our freedom". [2] According to this theory (as I understand it), the terrorists saw how free Americans were, saw how unfree they themselves were, became jealous, and attacked us. Now, there is no doubt that IF this theory is true, then the attackers indeed had no legitimate cause. But the theory (as I trust you will agree) is hardly believable, as it simply does not accord with our ordinary conception of how the human mind works. A more plausible theory of the motives is needed in order to make any assessment of whether the terrorists had a legitimate cause.

[1] https://www.democracynow.org/2019/11/15/headlines/analysis_f...

[2] https://en.m.wikipedia.org/wiki/Motives_for_the_September_11...