Stealing someone else's unused SSN is not synthetic identity creation, it's just fraud. There is a living person associated with that SSN, even if they are off the grid.

Synthetic identity fraud involves you making a bunch of inquiries with completely fake SSNs and squatting on them for a few years, maybe bringing them out of cold storage every so often to make a few more inquiries to keep them on the credit-building radar before you ultimately qualify for actual lines of credit and do a bust-out. There is no oblivious meatbag being used as a patsy-- the identities are completely fake, which is what makes them synthetic.