There are 2 important aspects of SDS - "Software" and "Storage", which distinguishes it from a FileSystem.
The software needs to replace the storage-management functionality of the hardware. Not all Filesystems replace storage-management - esp. the block level functionalities.
So, the SDS test is - Deploy the software on a commodity x86 hardware ( aka -"any hardware") and if one gets all the functionality of an enterprise storage hardware , then the storage software can be called a SDS solution.
Obviously, there is a grey level - storage functionality might range from block, file, object ..resiliency,optimization...etc.
However, a better SDS solution is one - which can add more storage functionality - on the same commodity hardware.