Wrong actually, it is indeed Quina. The only other Qu to appear in game is Quale who looks nothing like Quina, though Quan is mentioned but never seen and we know he dies before the game takes place because Vivi was raised to be eaten by him. If I remember the many hours I put in to this game correctly there is a part where this is explained. Quina was sent to the Alexandria castle kitchens to be head chef for the celebration as kind of a "visiting" gourmand chef however during this time the party do not know Quina and it is plausible that any of them who do see Quina (Steiner being the only one if I remember rightly) that they could have forgotten about Quina's presence or since Steiner seems more preoccupied with finding Garnet and Zidane never even noticed that Quina existed.
That or like the many other secrets Steiner seems to keep from the rest of them he keeps the information quiet in the hopes that he can use this information to seek help from Quina to return Garnet to Alexandria. It is highly doubtful being a princess and all that Garnet would ever meet any of the kitchens staff let alone a visiting chef. Most royalty never see any servants below the top level the idea of a servant is to be known of but never known by the master