That’s still not a great metric of ethnic cleansing. Relative populations of white people in Canada are decreasing, does that mean there is ethnic cleansing of white Canadians? Of course not. The same also goes the other way: an exploited group’s relative population increasing does not mean they are not exploited. Population levels are not on their own a very useful metric for trying to understand whether a group is being oppressed.

There are much better arguments to use that are clear indicators of oppression/systemic violence, since population on its own is not inherently correlated to racial discrimination. It’s so much more effective to point to, as you mention yourself, the exclusion of ethnic minorities from the one-child policy, or the increase in life expectancy, the elimination of absolute poverty, the increase in health care, cultural/religious protections, civic engagement, etc in Xinjiang (or other regions with large minority populations) to dispel myths about Chinese targeting/elimination of minority populations.