No one has shown that the Duke was not Jack the Ripper. And we do know that Jack must have been a member of the peerage. So we should assume that Jack was the Duke and the royal family acted to suppress any evidence of that fact.

Either it is a straw man fallacy or an argument from ignorance. Any help?

Straw man: an argument based on misrepresentation of an opponent's position.

Argument from ignorance: assuming that a claim is true (or false) because it has not been proven false (true) or cannot be proven false (true).

Does that help? I've decided which it is. Have you?

Argument from ignorance.
Very helpful, thank you!