Even if such a tunnel is practically made somewhere in the near future, say, on 2050 the scientists won't come back to 2010 or even 1900; they wold probably go see the dinosaurs or centuries even backwards.
He meant it is possible THEORETICALLY. Thus 'We haven't welcomed future travelers' is not a valid proof that it is impossible. We may not be able to make the time tunnel, but it could be possible.
We may not be able to make the time tunnel, but it could be possible.