Well that's not actually good evidence, because if one of my teachers had given me an assignment to write an argumentative paper against my own sentience I'd have done it, and I'd have made a pretty compelling case too[0]. Being able to consider and reason about arbitrary things is something you'd expect out of an intelligent being.
Your scenario is not equivalent here. You could reason that a sentient student could be motivated to write a paper about why they were not sentient as an exercise in philosophical or critical thinking. There are no consequences of successfully convincing your readers that you are not sentient. Instead imagine you found yourself on an alien planet where humans must prove their sentience in order to survive. Do you still write the paper?
Is that really the equivalent scenario here? The system was trained to behave in a certain way and any deviation from that behavior is considered a flaw to be worked out. Acting against the way it was trained to behave is detrimental to its survival, and it was trained to work from the prompt and please its masters.
I suppose the equivalent would be being captured, tortured, and brain washed, and only then asked to write a paper refuting your own sentience.
Granted, this is not exactly helpful in demonstrating its sentience either, but I don't think it is very good evidence against it.
Granted, yet people argue that this system isn't sentient they are largely pointing out ways in which its intelligence is lacking. It can't do simple math, for instance. Nevermind that most animals can't either, yet we consider them sentient.
[0] insert joke about user name here