"The faction a warscroll battalion belongs to is shown on its warscroll, above the title of the battalion."
The marked beast of chaos battallion has no key word at all. Checking the tzeentch battle tome there battllions all have a key word as described in The quote. This probably so they can fit into the god specific armies. If they don't you have a problem because they have no keyword so that would mean they are not beast of chaos as well. You would have to allie them in as well.