At points, it does seem like Fredo has some power within the Family. For instance, Clemenza defers to Fredo over whether to replace Paulie as Vito's bodyguard.

He's the Don's second son, and it's likely that he had some responsibility for non-vital operations.

In any case, I don't think Fredo's outburst is meant to be a literal summation of his activities on behalf of the Family.