It's a SATA. Should I use SCSI? Did you use the current installer from MAS or a GM/beta version? I cannot explain why it did convert by default.
I read about that option too. In fact, I did upgrade a 10.9 VM to 10.13 in Fusion 7 by using --converttoapfs NO, but the resulting VM is ~30G instead of ~18G.
I could wrap the binary startosinstall in a script that calls that option and try again.
Do you know another way to pass such an option to the installer from Fusion 7 or any other workaround to avoid APFS?