Since the clause about food from the underworld is boilerplate, it’s something that most of the gods know about, and Core is no different. Many attribute her fasting upon arrival simply to sorrow over missing her mother, but this doesn’t exclude the idea that she knew that she could not eat of this food. Hades plied her with food the entire time she was there, as he understood this to be the case.
The entire game has been about getting her to eat underworld food. If she eats, Hades wins. If she doesn’t, Demeter wins, and, as Demeter’s daughter, Core would know this. She couldn’t help but know this, especially as it is the standard.
This, of course, begs the question, why did she eat the seeds?
Simple, she wants to upgrade.
Demeter is adamant that she will starve out humanity unless she gets her daughter back. Core knows how much her mother loves her, and the math isn’t hard that no one side is going to win this entirely, unless it’s Core.
By eating the seeds, she irrevocably becomes part of the underworld. This cannot be changed. It can be mitigated, however. Because Demeter is adamant, Core must become a timeshare. Half the year she is on earth with her mother, and half the year she is with Hades as his wife. Had Core consented to marry Hades in the beginning, she likely would have only assumed this new aspect as queen of the underworld. If she had not eaten the seeds, she would have gone back to being a daughter of Demeter, a goddess of agriculture.
By taking the seeds, she must, of necessity in order to attempt to satisfy both parties, be both. She remains the daughter of Demeter, holding power over agriculture and the earth, and is the wife of Hades, the queen of the underworld, Persephone.
She now has dominion over the world of the living and the land of the dead.
Core, through this compromise, has become one of the most powerful goddesses in the Greek pantheon.