It is likely that Mojave will automatically force the conversion all types of boot volumes to APFS, including Fusion drives and hard disk drives (HDD).  Although things could change, this is the behavior being reported with the Mojave beta 1 release.

High Sierra was originally supposed to convert SSD and Fusion drives to APFS, but at the last-minute Apple decided to only force convert SSD drives.  One can only assume this was because of instability reported during beta testing.