In the real world, a d20 roll with mods generated from a 20 INT can do better than a d20 roll with mods generated from a 5 INT.Your argument fails to address one very, very important point that he made to you. Holmes is considered to be the best investigator. The pinnacle of what is possible. Your 5 int Holmes can become very good, but he can never, ever be the pinnacle of what is possible, because a 20 int PC with all of those ability will be better.
But the issue of whether or not Holmes is the best isn't a question about the real world; it's a question about the fiction. In the fiction, perhaps the 5 INT Holmes nevertheless is the best (due to the luck of dice rolls, the vagaries of fictional position, etc).