Chung was replaced a year ago with Phillips and possibly even Dugger. Bill and most people paying attention to Chung's 2019 play have known for a while. Even if Chung decided to play this year, he would have been buried on the depth chart and possibly even cut.
I actually think Mills might be more about CB depth considering we could be losing Gilmore, Jackson, and JMac all in one offseason (hopefully we keep the first 2 guys). I've heard Mills kinda sucks at CB compared to S, but it seems to be where he could see playing time the most easily.