My casual interpretation is that most kids didn't want to use the AI tutor. The authors estimate that time spent using the AI tutor is as effective as time spent studying without AI assistance, which is around one fourth as effective as studying with a real tutor. The real tutors are obviously quite expensive, comparatively.

So, if some kids do prefer studying with this AI tutor, that's as good as studying without it. Whichever approach gets them to put the study time in.