@kkoki So I'll just outline how I consider that a completely unfair point to make.
1. Just because it's a made up setting does NOT mean it can't reference real issues. Ever noticed how X isekai has everyone hating on demons even though they're actually really cool? That's the author creating a dialogue on racism. Addressing real issues is a huge concern in what, probably most people, consider "good" literature.
2. You, yourself, just called it "Muslim culture", meaning you fully understood and connected the issue in the manga to a real world culture and religion.
3. If you want to connect it to countries specifically; I think I can do that to a debatable extent.
So the story mostly has talked about the east/west empire/kingdom and cloyan. To me these countries reminded me of East/West/Middle Frankia of the medieval era (middle Frankia was later called Lotharingia and was short-lived and absorbed mostly by East Frankia) these countries later became France and the Holy Roman Empire/even later Germany. The Eastern Muslim place probably refers to something like the Ottoman Empire, who uppon conquering Anatolia, the Balkans, Egypt, etc in our world, they shut down many forms of trade with Christian Europe.