First, you have to figure out if it is the HU software/firmware that is limiting the rear camera resolution to low quality or the camera itself.
You can't assume that if you upgrade the HU to the latest in the 20MY, it is going to give you the same HD resolution rearview camera as on the 20MY Traverse. It could be that the rearview camera was upgraded as well to HD, either sensor or otherwise.
The rearview mirror camera is a separate HD, wide angle camera. The rearview backup camera is a separate camera but no full details or specs on it were really ever given by Chevy.
I also see in your screenshot that there are also several more camera views and options that you can choose from now. There might be additional cameras installed as well for those.