You either have not read the brief or you do not understand Rule 12(b)(6). Rule 12(b)(6) is different from Rule 12(b)(1) in that it does go to the merits, as the Supreme Court has pointed out, and says that on the merits a claim is not sufficiently made out, oh lurking liberi.
I know that well. But the brief is still wrong in claiming the defendants had to cross-appeal.