The Muslim influx -- Palestinian, Iraqi, Yemenite -- has come largely in the last generation
I think the author included this sentence here because he wants to emphasize how American tradition didn't have much of an influence from Muslim people for a long time, and hence traditions developed without considering Muslims as it wasn't much of a concern. Now, with the increasing amount of Muslims immigrating to Detroit, it is interesting how, as it talks about earlier on in the article, Muslim high school foot ball players have combined their fasting for Ramadan with their high school foot ball career which evolves their foot ball experience.