Well well well! That was a nice surprise, and also an interesting discovery I never noticed before.
First: Yes this does work for all 1080 framerates except for 120 fps. It zooms in in 0.1x increments, giving ten discrete zoom steps between 1x and the max, 2x.
Further, it is not just zooming the FPV in the remote display but is also recording the same on the SD card on the AC. Also it is clearly apparent that the local hardware/firmware is preserving resolution by cropping the full-resolution 4K sensor data to the zoomed frame and then scaling to 1080x1920 -- always decimating the image, thereby preserving maximum detail.
So, if you want a zoom function on Android, Litchi has it now. DJI must expose an interface for this in the SDK, Litchi's used it, but they haven't gotten around to porting the pinch-zoom implementation to Android yet.
In any case, this is a much better way to operate it. Zoom in on C1, out on C2. Much easier to focus on composition while zooming in and out.
Even better would be to hijack the right index-finger wheel for zooming. I'll contact Litchi and see if that can be done.
Second: Can't zoom in 1080, 120fps because the camera does a 2x zoom when you switch to 120fps!!! What's this? I never read that anywhere, in the manual, or in this bboard. Just checked it with GO and GO4 and it does it there too.
Why is it doing this? I have a guess: At 2x zoom the resolution is pixel-for-pixel 1080x1920. No decimation is necessary. This removes a significant bit of image processing, and therefore time for each frame. At 120fps, the system is just not fast enough to scale anything bigger than 1080x1920 to 1080 at that frame rate, so DJI just locks in a 2x zoom and records the pixel values directly.