“Islam” as a named religion with its own prescribed rituals and laws is specific to the message preached in Arabia in the 7th century.

“Muslim” is a more transcendent term that encompasses all the prophets mentioned in the Quran, as well as those not mentioned. Their creed and state of mind (absolute submission to God’s will) is the same, but they did not follow a religion called “Islam”. Earlier prophets and their followers prayed, fasted, and gave charity. Some even made the pilgrimage to Makkah. However, certain details of these rituals may have differed between them and today’s Islam, and between one another.

I don't see why "Islam" could not also be seen as a transcendent term, in that any religion that was revealed by God through a prophet or messenger is Islam, in so far as that religion conforms to His will.

Quran 3:19 Indeed, the religion in the sight of Allah is Islam.