This requires some clarification. Now, assuming you're talking about the entity known as "Old Man Zangetsu" (not the real Zangetsu) he's not exactly Yhwach. Look, Yhwach is the progenitor of all Quincies. All Quincies got their powers from him, and therefore his blood flows through them. That's why "Old Man" takes on the appearance of Yhwach. However, he is not Yhwach himself, as it is doubtful Yhwach would have grown to like Ichigo and allow him to get strong enough to defeat him.