At one point, the narrator says something like, "Who knows! Maybe Heathcliff's father could be a Chinese emperor, and his mother an Indian princess!" So in this case, he's probably a few shades off Italian.
People have no concept of history lol. They read "dark" or "brown" in a novel by an English woman from the 1800s and think it means 21st century American racial dynamics. Heathcliff was "dark" because he's like half Romani or Italian. They were so white that Italian was ethnic lol