Had the comment been made on the record by an official government spokesperson Shipman would have been well within his rights.
The spokesperson would have been accountable for her or his allegation against Rudd. He or she could have been identified and questioned about it.
Instead Shipman allowed an unknown Whitehall figure to label Rudd a liar, while granting him or her complete impunity.
Put another way, he allowed his Twitter account to be used as a vehicle for someone unknown to smear a prominent public figure as dishonest.