My theory was that they are former legendary Marines/cipher Pol agents who were made immortal by Law's fruit for their service to the World Government
If we take literally the spoilers the way they say "they werr always part of history", and if that shanks lookalike os from figarlan family too, and if we assume that family is part of the CD's, then these 9 are all from the CD family, made immortal by capturing, manipulating and sacrificing ope ope no mi users in the past to make then immortal.
Imolications from that might be they were the warriors who took part on the Ancient Kingdom's demise, part of the void century. Maybe there were 19 of them but 10 of them got beat during that time by Joyboy and his allies, one from each family, but not buying fully into that.
Perhaps the system at that time was entirely different, they had soldiers/army/knights etc., less modern thay current OP, with no marines, pirates etc. Tho they could have had spies at that time which are equivalent of cipher pols to this era.