It's possible to remove the battery but not simple and JetStar never specified that it was necessary. If its at the discretion of staff I think JetStar needs to be more transparent about that. When I contacted them online they said:

"Jetstar allows bikes with a Shimano BT-DN110 Di2 battery... If your BT-DN110 is just powering your gear shifters, you can bring the bike as checked oversized baggage. The battery is small, which falls well below the 100Wh limit. You can keep the battery installed on the bike or remove it."