It's also possible the flight attendant was not a DL flight attendant, but one from another airline opting to list for a jump seat cabin agreement. This is an agreement between airlines to allow other FA's to travel reciprocally for free if they don't have direct benefits with the airline they are flying on (like a Spirit FA trying to fly on DL metal). Doing this gives them the lowest standby priority, but it's free and an alternative to using a ZED (Zonal Employee Discount) fare. DL doesn't typically allow FA's to sit in FA on this type of agreement, but ultimately the gate agent may have had a moment of being nice when they shouldn't have and gave it anyhow.