This really depends on several factors.
Halo Silentium Page 181/182
“The story told by the walls and in my flesh was simple enough: Ten million years ago, Forerunners did indeed travel to Path Kethona. We came to finish what we had started in the home galaxy: the total destruction of the Precursors. They had judged us and found us wanting, they had chosen others in our stead. We were not in line to inherit the Mantle. Ans so we began our purge, and in Path Kethona, did our very best to finish it.
We were not then powerful enough to erase all evidence of the Precursors, to destroy their star roads and citadels and other artifacts. And so we left at least one Precursor behind, to live out dreams of vengeance and hatred, to lay down plans in cold and darkness at the heart of a lost asteroid - over millions of years.”
This seems to imply that the Forerunner left the Primordial behind in Path Kethona. It’s possible she actually meant they left him on that planetoid where the Humans eventually found him, but this would contradict the Gravemind’s statement that the Primordial arrived in our galaxy “nine million years before.”
To me it seems like ten million years ago the Forerunner defeated the Precursors at Path Kethona and buried the Primordial in an asteroid. Over the course of a million years that asteroid, or maybe just the capsule the Primordial was in, returned to the galaxy where it was eventually discovered by Humans.
We know more than just the Primordial survived as the others became the powder and that powder arrived in our galaxy an unknown number of years ago. If the Primordial did return to the galaxy from Path Kethona, perhaps that means they arrived at the same time as he did? This would mean they launched at the same time or around the same time and both arrived “nine million years before” like the Gravemind said.
Forthencho believed it was the Primordial who unleashed the Flood, however it was pointed out that the Flood had already attacked before his discovery. But all that means is they discovered and unleashed the Flood before they found him. He still could have been in contact with the unleashed Flood before his eventual discovery.
The Primordial itself was described as being more than one entity. He may have merged with other Precursors to form what was found in the Capsule. The first of the entities which would come to be called “Graveminds”.
The Didact before had said that the Gravemind he faced was new.
Halo Silentium Page 172
“An awful, awesome mash-up of Forerunner and other creatures, gathered from across the ecumene, more confused and even more disgusting, if that was possible, in its somehow physically younger, but conveying all the ancient knowledge and power of the Primordial. This was new. This was still very, very old.”
The Master Juridical thinks about this after hearing what the Gravemind had to say.
Halo Silentium Page 175/176
“Intriguing! The Bornstellar Didact took vengeance upon a Primordial before Juridicals could investigate. We have no way to gather testimony from that remarkable being. But Catalog in the Burn has connected with a Gravemind, which, by its own admission, appears to be the nearest equivalent.”
The way I see it, the Primordial was a creature which we dubbed “Gravemind” but a much much older form of Gravemind. The one that appears after the Primordial’s demise and the one we see in the games is like the “new model.” This is what is meant by “nearest equivalent.” He is new but he is just a newer form of the Primordial which preceded him. It formed from the powder which became the Flood and then carried out the Primordial’s vengeance with his complete knowledge and power.