The war rig is sort of like Furiosa's way of testing compatibility: if you can grasp the importance of that sliver of her homeworld, you stand a chance at understanding her. So Max nods, and his expression loses some of its edge. "Means she likes you."

His personal opinion is that Furiosa likes people too easily, but there's so much that's familiar about Rey, so much that he can read and understand, that he thinks Furiosa was right to show Rey the rig.