Tim Staples has said in the past (I heard him say it on Catholic Answers Live) that the title “Co-redemtrix” is not a title that is dogmatically defined. He also said that in his opinion it will be so defined soon by the Church. This probably explains why he feels free to use it (along with Redemptrix, another title not defined dogmatically but one he feels will be soon).
To be clear though that’s his opinion. I tend to agree with him (that it will be defined dogmatically soon) but it’s not yet dogmatic. So Catholics are free to disagree with applying that title to Mary if they wish.
Typical Catholic wanting it both ways stuff.