Inductive reasoning made me jump to that conclusion. Two chapters ago Doflamingo claimed he’s been locked up and possibly secretly assassinated because he knows of this “hidden treasure” in Mariejoa. We’re lead to believe at the end of the chapter that it’s the oversized [Joy Boy?] straw hat that this silhouetted figure is gazing upon that is the treasure.
This is a clear misdirect by Oda because later on Stelly’s manservant provides us with the exposition on what the empty throne surrounded by ancient swords has meant to all the allied nations for the past 800 years, so when Ym sits in front of the prostrate Gorosei after he/she was looking at stabbed bounties and headshots of D-clan members and Shirahoshi aka the ancient weapon Poseidon it cements the idea that this is a longstanding figure with an immense grudge.
Secondly, Doflamingo’s knowledge of this character would also explain why he’s aware of the true ability of the Ope Ope no Mi and why he went to great lengths to procure it. This secret to immortality wasn’t some common legend heard through the grapevine, this was ancillary knowledge he obtained through finding out the existence of Ym. Ym being around this long is most likely the result of this procedure.
This logically explains why the Marines (through the orders of the Gorosei) were so eager to get the fruit from pirate Diez Barrels during Law’s flashback story, and also why the unwitting Barrels himself wasn’t curious as to why the Marines were so willing to pay a whopping 5,000,000,000 beri for a non-Logia Devil Fruit.
Lastly, the disposition of Doflamingo and his subservient inner circle (both of which counted 5 members before Rosinante’s demise) reflects the relationship between Ym and the Gorosei. It seems clear to me that Doflamingo is trying to take Ym’s playbook and his own family’s history to restore himself to prominence, which would’ve been a complete imitation if he acquired immortality.
So when you add all that up, it all points to Ym.