First of all:
Second: Understand the mechanics of how Upwork's tool works:
Third, you need to have a correct understanding of what is happening, and what your obligations are.
It looks to me like this client is just trying to get money from you.
(It is pretty obvious that the client is just trying to take money from you because the project was "account recovery", and you actually helped him recover his account, and then his "reason" for asking for his money back is that "your work is fake." Well, that makes no sense. If he hired you to write a story and then you plagiarized the story, then I could understand why he would call your work "fake." But if he hired you to recover his account and you actually did that, then where does "fake" come into it? Maybe this client is simply in the habit of hiring people to work for him and then accusing them all of doing "fake" work so that he can bully some of them into giving money back.)
If you did the work, and the work you did was honest and true, then you do NOT owe the client any money. If the client hires a freelancer to do a task, and freelancer does that task, then the client needs to pay the freelancer for the work, as agreed.
It looks like you have ALREADY been paid, and the contract is ALREADY closed, and the client clicked a button to request money from you.
You don't need to send him money. But keep in mind this: Sometimes clients just want to be heard and understood. I recommend that you DO NOT argue with the client. DO NOT tell the client that you are not going to send him money. ASK the client questions. Try to gain a better understanding of the client's concerns.
You ALREADY asked the client why he is asking for a refund. That is good.
You did the right thing.
If the client DOES NOT RESPOND to your question, then it means the client knows full well that he was just trying to take money from you, and he is embarrassed about what he did. You don't need to do anything further.
If the client DOES respond, then thank the client for his response, and continue the conversation, in a very thoughtful manner, which will take TIME in order to be done correctly:
"Thank you for answering my question. I am going to think very carefully about what you said and reply within two days."
Then wait two days, and send the client another very specific question, like:
"You said that you didn't like the D-21 file. Can you tell me specifically what you didn't like about that file?"
I wonder if he "account recovery" was merely temporary...
I don't think infuriating the client by telling them you'll "carefully think about it" is going to work.
The client has 30 days from the date on the invoice (which you can find in your trasaction history) to dispute the contract.
I'd not play games and keep the communication open politely and professionally.