My bet is CASA want Tiger to take control of the initial endo training. ie no more pay up front and get it where you like.
If that was the case then J* and Virgin would have to meet the same requirement
I believe there is a subtle difference. Isn't it the case with Virgin that a prospective pilot is compelled to do their endorsement at Alteon in BN because Virgin have an arrangement with this provider. I don't believe you can just help yourself to a 737 endorsement anywhere and start work for them. Although that begs the question that a pilot who is A320 endorsed goes to work for Tiger, they also do not go through a provider aligned with the AOC holder.
I doubt that this would be the major sticking point, it is more about the standardisation issues once inducted into the airline regardless of where the pilot got their endorsement.