Correct. He is a separate unit when attacking, and thereofre must be in
btb with an enemy model in order to fight. As usual.
When firing flamers from an open topped vehcile (burners in a battlewagon im guessing

) as long as you can draw
LOS from the hull to the target, ALL templates fire from exactly the same point - the point at which the most models are covered.
This means that you dont actually need to place the template entirely in
LOS - and in fact can be forced not to.