The hypocrites did not merely utter the proposition “You are Allah’s Messenger”; they introduced it with نَشْهَدُ (“we testify”). This matters for the replies to the argument.
The proposition of prophethood is true, whereas their act of testifying may itself imply an inward conviction and commitment that they lacked. The charge of lying can accordingly be directed at the profession of testimony rather than at the true proposition affirmed within it. The lesson stresses specifying the target of the truth assessment.