Exactly, but it could be easily proven that the data was tampered with as you still have the original/clean image and the device itself. So if they are altering data, it can be shown.
If the phone could have been altered to add data before a forensic image was acquired in a way that looks like it was via normal use (reasonable timestamps, browser logs, etc.) then we'd have a real problem. Cellebrite potentially being used to do this as it reads the image is not that problem.