Can it be said that Jibril (Gabriel) - peace be upon him - was not solely responsible for revelation, based on Allah's saying in Surat Al-Mursalat: "Then those [angels] who deliver the message"?

The noble verse does not explicitly state that the descent of revelation is not exclusive to Gabriel (peace be upon him). The intent behind "angels" might be Gabriel alone, with the plural form used for glorification, or it could refer to the guardian angels who accompany him.