Abstract from your link suggests that the study was pretty inconclusive.

> Findings suggest certain flaws limit the program’s potential success since the program design is misaligned with its primary stated goals, and several program goals relating to external effects of fare reform cannot be evaluated. Although it would be valuable for transport managers in other cities to learn about this experience, the Tallinn fare-free public transport program provides scant transferable evidence about how such a program can operate outside of a politicized context,