It's dehumanizing, but also in a slightly different sense than you recount here. It's dehumanizing because it takes agency away from the person who has achieved that positive quality. A hard-working Asian person is hard-working simply because they are Asian; it's in their blood, it's natural, and they deserve no real commendation for being hard-working because it is inevitable, it is out of their hands. An athletic black person's sweat and skill is simply their biological destiny, not anything particularly praiseworthy.
Model minority-hood is in fact a comparison with white people, because, in this sense, white people have no inherent positive qualities, and so they have "earned" all of the attributes they manifest. Hard work is hard work earned for white people, not a passive part of their inherent identity, so the individual is to be commended for it. "Positive" stereotypes remove any agency from the individual so that anything they might do, even things that are unique, amazing, or positive contributors to society, are not really quite so valuable as if a white person did them. This is the harm of "positive" stereotypes--in reductively "lifting up" the group, they devalue the accomplishments of actual individuals. They reduce whole populations to biology, and fail to recognize the humanity of people of color.
