I started reading this paper and found this important bits:

> During the study period (3 April to 2 June 2020)

This is the period where almost all of Europe saw a decline in daily-new-cases, mainly due to people starting to take this thing seriously.

And still, the study has shown that masked worked:

> The primary outcome occurred in 42 participants (1.8%) in the mask group and 53 (2.1%) in the control group.