Ganondorf is an incarnation of Demise that's what he means much like Zelda is Hylia's descendent in the same way, Ganondorf is actually Ganon (an incarnation of Demise) taking on the form of a Gerudo with the Gerudo male effectively becoming an host/avatar of sorts for the dark beast. This is why Ganondorf looks like Demise and has an affinity for the Triforce of Power.
So basically Demise saw a new Gerudo baby boy yet to be born and decided to take his body for himself?
Something like that it's never fully explain, Ganondorf's mother was a Gerudo witch or to be accurate twin Gerudo witches (The Twinrovas in OOT) who were in their singular fused form when he was born it's unclear if this part of a ritual for Ganon to be reborn as a Gerudo. All we know Ganon and the Gerudo boy are and can be separate entities because in WW Ganondorf no longer has the dark beast while various other games have shown Ganon to be its own thing and even have different hosts/avatars.