Look at their graph of monthly data.
https://i.imgur.com/mc8PJW8.png
You can't draw any conclusion from that but that it is an overall increasing trend with cyclic tendencies by season.
March prior to the release in the end of March was already all time high rate of suicides.
"Table 1. Association Between the Release of 13 Reasons Why and Suicide in the United States, by Age Group"
I mean ... it's just stupid. The series' premiere date is also a arbitrary point in time.
I don't refute that the series might have increased the suicide rate, but I have no idea how to verify that or put a estimate on it. The authors surely don't know either.