Do you really expect Biden to say in this very moment "I will investigate and lock him up" ? Federal charges were always a moot point, since Trump can pardon himself. Its the state AG's that can really do something.
He could have just said he'd leave it completely to the Justice Department to determine any investigations. That would have struck a neutral tone and allowed justice to be carried out. Instead he's taking a stand, the same stand Obama did with Bush, and it sends a horrible fecking message that justice doesn't apply to the powerful.