The LCC model favours one variant of one type - ie. only 738, or 737, or A319, or A320 - all with only ONE config. That way, your other systems are kept very simple - you always know what the cost of operating every single flight is, you don't have to worry about changing crew numbers to operate different a/c, etc. Then you manage your demand by having a dynamic yield management system which changes fares as demand changes.
So logic suggests that a real LCC would not go for even a different variant of what they operate.
So it'd be illogical for JQ to get 321s.
So it's probably a dead-cert, then...
