There are several problems with this claim. For starters, yes, for centuries Jews in both the Islamic world and medieval Europe were “a nation without a country” and didn’t “assimilate”, but that wasn’t by choice.
They weren’t allowed to assimilate, were drastically restricted in the businesses they were allowed to run, wear they could live and even what they could wear.
So yes, the Jews of Morocco and Iraq didn’t “assimilate” but when they weren’t even allowed to live next to Muslims, had their houses marked with signs identifying them as Jews, were required to wear specified distinctive clothing, and weren’t even allowed to ride horses of course they didn’t assimilate.
The same was true of the Jews of Poland and Russia, though they were also treated vastly worse than the Jews of Morocco and Iraq who at least had a legal proscribed rank in those societies, though it was a very low rank.
In the places where they were allowed to assimilate(France after Napoleon opened up the ghettos, modern Great Britain, Germany, and of course the United States) the overwhelming majority did assimilate.
The Jews who refuse to “assimilate” in the US aren’t “orthodox Jews” but Hasidic Jews or “Ultra-Orthodox Jews”.