Actually, it was never intended to be a sequel. It was just seen that way because of the naming fiasco over in Europe. It was released as "Zombie 2" in most of Europe (as "Voodoo, Schreckensinsel der Zombies" (Voodoo, Horror Island of Zombies) in Germany, and "Zombie Flesh Eaters" in the UK) due to the popularity of Dawn of the Dead, which was released as "Zombie" by Argento. So they wanted to capitalize on that.
I thought Zombi(e) 2 by Lucio Fulci, was originally supposed to be a sequel to dawn of the dead, but the studios made them change the name and advertising because Day of the Dead was in production already.
I think that's pretty much just something that people have said over the years, and now it's taken as fact. If anything, if you go by the story's timeline, it should be a prequel, not a sequel, since it shows how the zombies came about.