If she didn't send that $200 voucher in to Oceania, they aren't going to honor it.
What is more likely,
Your TA made a mistake, understandably doesn't want to fess up, but made you whole by giving you $200.
or
Oceania won't honor their own coupon which they send out by the tens of thousands?
It could be either one. I'd lean towards the former, with no malice towards your TA, but reasonable people can disagree on this.