What is the testimony that was concealed by the polytheists in this āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. Allah bore witness to them with a testimony that Abraham and those mentioned with him were Muslims, but they concealed it. [al-Ḥasan, Zayd ibn Aslam]
2. They concealed the truth about Islam and the prophethood of Muḥammad, while knowing that he was a prophet and that his religion was Islam. [Abū al-ʿĀliyah, Qatādah]