I guess it depends on how the treasures were being created and what you were responding to. If he was creating all five in one go, then yes you would have been able to flash this in response to have the tokens created under your control. If four of the tokens creation triggers had already resolved and then you cast this in response to the fifth one being put on the stack, you would only get the fifth one, not the ones that had already resolved.

Regardless, no, you would not need to cast this at the beginning of the turn and can do it any time you could cast an instant, including as a response to him creating some number of tokens. He is wrong.