As I don't have a Fusion Drive to experiment on, I cannot test or otherwise verify this, but Fusion Drives are virtual entities somewhat along the lines of APFS volumes. I believe that in order to run a Surface Scan you would have to address only the HD portion of the drive.
NOTE: Beginning with High Sierra (MacOS 10.13) Apple does NOT permit a surface scan of the boot drive.
If we knew what it was we were doing, it wouldn't be called research, would it?
