Like probably many people have already commented on the post there is a likelyhood that both Cogita and Volo are Celestica people - not descendents but immortal survivors. Which would fit extremely well with AZ (the immortal giant king from Kalos) and would establish a theme of immortal survivors of the ancient civilizations that have long since perished.
BUT I would like to add onto that:
First off, a bit of context: Sinjo/Hisui/Sinnoh as a region is based on Japan’s Hokkaido. Mount Coronet is not - as many people I’ve talked to assume - based on Mount Fuji but rather the Mount Yotei (also nicknamed the Ezofuji for its resemblance to Fuji-yama). Added onto that base template of Hokkaido were elements of classical Ancient Greek architecture and mythology. (with a bit of spice from Latinate Rome)
What you will note is that Mount Youtei, and therefore Mount Coronet, is an still active volcano. And more than that, just look at all the calderas and craters scattered accross the region AND the island that is literally a volcano. Hisui is an active volcanic field. So, let’s take a look at Old Verse 20 and Old Verse 2:
“Once it shone upon us all,
with all the warmth of welcome sun.
But now we weep, to grief we fall,
starved of light now it has gone.
And some they go, despair withal,
in search of it they reel and run.
They quit their hearths, abandon hall,
and leave our lands to be undone.
And when they’re gone beyond recall,
this land will be a home to none.
This land will only ever be a home to Pokémon.”
Oh you, who at the world’s far-off end dwell
I know your wish - it is my wish as well.
But ours are cold and endless winter days,
warmed only by memories locked away.
The end of the Celestica people as a civilisation was a volcanic eruption. An eruption so monumental, possibly of several vocanos simultaneously, that it caused the ash in the stratosphere to block out the sun and likely cause a lasting volcanic winter in turn.
Without sunlight the Celestica people leave the region in droves and they scatter accross the world. And as the poem says once all the original inhabitants have left and the few remaining people in Sinjo die, it will be only a “home to Pokemon.”
With all that out of the way, what I’d like to propose is that Cogita was part of the Celestica people in their prime. She was educated, learned the culture, the history, the religion and tales of heroes. Eloquent and well versed, she recorded what she learned and experienced in poems.
Volo, on the other hand, was born and raised at the tail end of the Celestica civilization. He grew up in a permant winter, bereft of sunlight. A place where even seeing another day was never a guarantee. All he knew was an abandoned land filled with empty towns and empty shrines. A cultre that was dying that he could not understand beyond what little his parents might have imparted of their faith. Silent statues of saviours and guardians staring down at him but not easing his torment. Hardship and suffering. Alone, and with no understanding of why he is suffering.
It’s obviously unknown by which means these two would have gained immortality. Maybe it was a boon, a blessing or a curse granted by Pokemon of Legends. Maybe it was the hubris of the Celestica people that turned one of them immortal while causing the chain of volcanic eruptions, similar to AZ and his Ultimate Weapon.
Either way, Volo thinks himself Celestican, carrying their blood even when their civilisation was already in decline when he was born. But Cogita thought him to be a decendant - another claiming “the name not theirs to take” (Old Verse 5), not someone in the same situation as herself.
It i quite the tragedy, isn’t it? To have the only two survivors of a civilization forever forced to go on living but unable to find comfort and kinship in one another because of their vastly different experiences. Although… I do find comfort in the hypothesis that Volo and Cogita go on to eventually become Cynthia and her grandmother. To believe that in time Volo will find an identity that fits him alone, as a woman that conducts research not for the The Great Unknowable (aka The Almighty Sinnoh aka Arceus) but for herself. And after traveling the world - expanding her views on the world and its history, taking time to appreciate living in all its facets - she returns to Sinnoh and finally finds a familial bond with Cogita. No longer alone. Neither of them.
Awaiting the day a certain soul finds their way to her, and to help them grow like they once helped her.