Hello! they likely constrained the black pupil bone to the bone that holds the iris and eye reflections with a setup pose exactly in the middle. They then set the transform constraint mix to 110 for the translate value, and 60 for rotation.
Transform Constraints - Spine User Guide
If you're still confused you can post your project to check what to change to get it to work.
Here's also a similar example that uses weights and no constraints, this time by weighting the iris to the eye (which is not moving) and the pupil (which is controlling where she looks) to achieve that effect. Spine: Tips: 2 eyes
