You are right actually, it's deaths that reduced for another 6 weeks after mandation although they were very minor figures anyway. Either way i've no doubt these people associated masks with low rates judging by what I was reading at the time on social media etc.
A cynic might think that the government chose to introduce them at the time they did because they could reasonably predict what the figures would do, given the time of year, and knew that this would be seen as evidence that masks work!