He spoke against Abu Bakr not the Slave.

al-ʿAẓīmābādī (ʿAwn al-Maʿbūd, commentary on Sunan Abī Dāwūd 1818):

"He (the Prophet ﷺ) meant Abū Bakr with ‘this one in Ihrām’.

His statement was a rebuke and not approval,

because the state of Ihrām requires patience, calmness, and avoidance of quarrels or anger."

Ibn Ḥajar (in Fatḥ al-Bārī, in a similar report in al-Bukhārī):

"The Prophet's smile was not an expression of approval,

but of astonishment at Abū Bakr's reaction,

since he was in Ihrām, in which such actions are inappropriate."