A fair part of the Quran is rhetorical commentary on Judaeo-Christian religious traditions. I'm sure we can agree on that.
The Quran assumes its audience is so familiar with the Biblical narratives that it rarely makes any attempt to explain stories and characters before offering this commentary though.
These would make no sense to a Pagan audience completely ignorant of even the most basic aspects of the previous religions.
Other than the unreliable later traditions say so, why would anyone believe that the Quran emerged in an isolated Pagan backwater that magically managed to avoid coming into contact with what had been the dominant Abrahamic religious traditions of the Arabian peninsular for centuries?
If you go to any airport, you'll see plenty of people from across the Globe passing through. No one need know how their Country came into being or who key rulers were etc. For the most part they are just passing through, visiting family or conducting business. Similarly we are told Ishmael's descendants settled in Arabia and initially worshipped Allah swt alone at the sacred Mosque in Mecca. Over the generations fewer and fewer people stayed with Monotheism and as people traded with distant places of commerce, so too they began bringing back other cultures, ideas and gods. Paganism soon took over and as you know, Pilgrims from many different faiths, beliefs and cultures would come to the Mosque and offer prayers and sacrificial offerings to their gods. No doubt some people would have known the basics behind the various faiths, but certainly not the finer details.
Muhammad pbuh would invite Jews and Christians for discussion and to reach common ground, hence the revelation of past stories from Allah swt. We know too his wife Khadijah, may God be pleased with her had a Christian Cousin Waraqa, who was a Monk of sorts, so would have been on hand to explain some details, though he passed away shortly after Muhammad pbuh started receiving revelation.
Aside from knowing there were Jewish tribes in Medina, awaiting the Prophet mentioned in Isaiah 42, there is no real evidence showing thriving Christian and Jewish communities elsewhere, perhaps some small settlements, but nothing like Yemen, which was under Christian rule.
That part of the World was much like a backwater, and we know this from the lack of mention in most Historical records, except from a mention here and there, and even then it was second an information passed on by traders.
You may recall from another discussion we had:
Historian Theodor_Mommsen in his book The Provinces of the Roman Empire: From Caesar to Diocletian mentions mentions that Aelius Gallus sailed with 10,000 legionaries from Egypt and landed at Leuce Kome, a trading port of the
Nabateans in the northwestern Arabian coast. He then conquered without difficulty Iathrib (Yathrib/
Medina). From there the plan was to crush any resistance and march down to Yemen, but was forced to abandon those conquests -according to Mommsen- not only because of diseases and epidemies, but even because he had overextended his line of supplies from Egypt in a land full of deserts.
It would seem the desert pagans, getting by with trade and caravan raiding never had rulers from the West or East presiding over them. It's almost as if GOD had a special plan for them.