inorganic chemistry - Why is the reaction of calcium oxide with sulfur dioxide to form calcium sulfite not a combination redox reaction?

I understand that a combination reaction is one where two substances combine to form a third substance, like with $$\ce{2Na + Cl2 -> 2NaCl}.$$ I do not understand why $$\ce{CaO + SO2 -> CaSO3}$$ is not a combination redox reaction. After all, it is creating a third substance, so why is it not a combination redox reaction?