KU does not sell F class on that route only Y and J. The so called Premium Y is a regular Y ticket you bought but you were placed in the forward of the Y cabin, which places you in the J seats. Hence the meal, and the Y service.

The J seats you flew on are the New F seats, and are sold as F to other destinations where F is offered. On this route, F is used as the J cabin, which is why you were seated there.