I'll believe it when I actually see Naruto dead. I don't believe that he is dead and quite frankly, I don't want him to be dead. Perhaps he is trapped somewhere. It doesn't explicitly say he was killed; it just says Boruto will be joining Naruto. This could be for shock value. Do not assume Naruto was killed off, guys. He could have been thought to have been killed for all we know but is still alive.
(Also why does no one talk about how Boruto seems to have Sasuke's sword and gear? What if Sasuke is in trouble too?)