I’d be taking this up with the TIO. If I’m entitled to a refund, they should be providing it and leaving me on my plan.
The reason for the refund is that they were guaranteeing the speed beforehand hence the ACCC intervention. They even tell you that you weren’t getting the speed you were paying for in their letter by providing an example.
If you now know how the plan works, you should now be able to stay on the plan and get the theoretical speeds you get based on this (is. No change). Up to you but I’d be fighting (even if they gave a credit on your account…)