I've been thinking why didn't she keep the ingredient a secret and told them it was sweet ...

I've been thinking why didn't she keep the ingredient a secret and told them it was sweet potato after they've tasted it??
Honestly those nobles have no common sense, just because it's a commoner food doesn't mean you'll become a commoner after eating sweet potato. Commoners drink water too, I don't see anyone not drinking it and calling water "commoner water"