A popular theory about King was that he was the former warden of Impel Down before Magellan, but the most recent chapter shows us that King was already part of the Beasts Pirates one year before Shiki's escape.
23 years ago is when Kaido and his crew fought against Moria and his crew at Wano, and as you can see in this panel King was already part of Kaido's crew.
One year later and 22 years ago from the present storyline that's when Shiki escaped from Impel Down, and as you can see there was a warden before Magellan.
So from this timeline it would mean that King was not the former warden of Impel Down before Magellan.
23 years ago is when Kaido and his crew fought against Moria and his crew at Wano, and as you can see in this panel King was already part of Kaido's crew.
One year later and 22 years ago from the present storyline that's when Shiki escaped from Impel Down, and as you can see there was a warden before Magellan.
So from this timeline it would mean that King was not the former warden of Impel Down before Magellan.