Slavery has a definition and legal sanction. Kidnapped people are not legally slaves or people held under fraudulent pretenses. Remember in those days there were debtors prisons and people very jailed for defaulting on debt.
As to practical difference to any in that condition there is none. De facto they were slaves but de jure they were not merely victims of crimes.
That makes sense to me now.
I was thinking more in the general sense of anyone who's liberty is revoked through no consequence of their own actions but through the criminal endeavors of another.
But legally a slave was a very specific thing and I guess these people were something else.