I am assuming the tilde above an object implies taking a transpose. If that is the case, then M is indeed a symmetric matrix. One can see this by looking at the Lagrangian. Since the Lagrangian is a number, we can take a transpose of L and we'll get back the same number, i.e. [itex]L^T=L[/itex], this will rewrite the Lagrangian in terms of [itex]M^T[/itex]. But since these are equal, we must have [itex]M^T=M[/itex]