Why is the following reaction not considered a redox reaction? The following question appeared in the Cambridge International AS and A level Chemistry Coursebook:


  Choose from redox, substitution, elimination, addition and hydrolysis to give the type of reaction for the following:
  $$\ce{C2H6(g) + 2Cl2(g) -&gt; C2H4Cl2(l) + 2HCl(g)}$$


The answer given by the answer key was substitution since $\ce{Cl2}$ replaces $\ce{H2}$. 

However, why is redox not a suitable word to describe the reaction? After all, the oxidation number of $\ce{C}$ in the reactants is $-3$, while it is oxidised to become $-1$ in the products. Furthermore, the oxidation number of $\ce{Cl}$ in the reactants is $0$ while it is reduced to $-1$ in the products. 

Hence, by that logic, isn't the reaction also a redox reaction since $\ce{Cl}$ is reduced and $\ce{C}$ is oxidised? 

Thanks in advance for any help.
You are right. The question is definitely misleading. If it would ask for the mechanism, (radical) substitution would be the correct term, as a reduction does not say anything about the mechanism. But "type of reaction" can basically mean anything, and in this case both answers are correct.