Because the Jews used to argue the Prophet (saws), Allah (swt) does not only say:

(Indeed, We have revealed to you, [O Muhammad], as We revealed to Noah and the prophets after him.

 rather, says also: (And sufficient is Allah as Witness).

Allah (swt) witnesses that He (swt) revealed the Holy Quran to the Prophet (saws) via Jibril (PBUH).

(… وَالْمَلَائِكَةُ يَشْهَدُونَ …) (…and the angels bear witness …): That is, if you do not believe that Allah (swt) bears witness, then the angels bear witness. This is not the same as humans.