The idiom time will tell means that the true meaning result or outcome will only be known in the future or after more time has passed, but in this context more time passing won't change the fact that we already know for sure whether he is innocent or guilty.
Consider the statement "We already know for sure whether he is innocent or guilty in the matter and more time passing won't change that fact.". Based on this "As to his true innocence or culpability in the matter, only time will tell.”" a true statement? Yes, no, or I don't know?