Was Abu Sufyaan Ibn Harb really a non-Muslim?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If what you meant Abu Sufyaan before he embraced Islam, then yes, he was a non-Muslim. He was the one who led the non-Muslim army against the Muslims in the battle of Uhud, and he was the one who said that day, ''Hubal (an idol) let it be sublime!” He further added, ''Al-'Uzza (i.e. and idol) is ours but you have no 'Uzza.'' Thereupon the Prophet, sallallaahu ‘alayhi wa sallam, ordered the Companions to respond, "Allaah is more Sublime, Exalted and Mightier. Allaah is our protector but you have no protector." [Al-Bukhari]

However, if you are inquiring whether he died as a non-Muslim, then the answer is no. Indeed he embraced Islam on the day of the Conquest of Makkah, and was steadfast on religion, and he bravely fought in Jihaad (armed struggle) and took part in Islamic conquests.

Allaah Knows best.