That's just basic probabilistic reasoning. If, say, 90% of people attacking position X belong to group A, and 10% belong to group B, it's safe to guess based merely on the observable "So-and-So is attacking X" that So-and-So is probably a member of group A (P(A|X)=90%). If 90% of people attacking position Y are members of group B, and 10% are members of group A, then observing someone attack both changes the probability to even odds (P(A|X,Y)=50%).

So, yeah, if I see someone attacking a view popular among the left, my first assumption is going to be that the person in question leans right.