Marx made the distinction that some people show up to work every day for their wage, but that their labor goes into the logistics of distribution of capital, not into the creation of surplus value.

Whether managers fit into that may depend on what kind of manager you’re talking about. Managers have been historically excluded from unionization drives not because of some intensive class analysis but because they’re used as the front line of defense against union pushes. Personally, I don’t think it’s in the class interest of most low level managers to do this, but whether they’re class traitors or whether they’re PMC doesn’t make much of a difference to me if they’re spreading union busting propaganda.