Basically Telstra will take any device for trade in. even if its damaged and not in the list. Time to trade in a dud phone i guess.
Eligible service required. Credit applied by second month. Trade-in powered by Kingfisher.
For reference Galaxy Z Flip4 128 GB is $62.45 on 24 month plan.
You will need a plan as Credit will be applied "credit to your bill within two billing cycles, or onto your bank account or credit or debit card for your AutoPay arrangement with us within 30 days, if you’re on one of our upfront mobile or data plans."
Trade in Terms
So you are pretty much getting $400 knocked off the new decide value.
You might be able to get that off from Samsung education or epp store. Understandably that's not for everyone but education access is one of the most common ones.