Board Thread:Questions and Answers/@comment-39573043-20190524150545/@comment-33880220-20190628002335

When you hit those user ranks, it would have said something along the lines of "Your cats can now be upgraded to level MAX+10, or something along the lines of that. What this implies is that, no, you can't use XP to get them to level 30, but yes, it does increase the amount of levels you can give them through cat capsules

In short, it only increases the amount of levels cat capsules can upgrade them. Not XP.