Suicide does indeed have multiple causes and if one of those undergoes a lessening of its impact, then that will cause a reduction in the suicide rate.Suicide can have multiple causes. Even a change in the weather could have effected the rate.
Plus, it's possible that e.g the change in rate is completely different from group to group, e.g. depending on the particular age group. High-school age versus college age, versus early school leavers, etc.
The study is simplistic to say the least, and a little too convenient in terms of the politically-perfectly-correct results.
Always a good idea not to pin your beliefs on a single study -multiple studies especially as regards the 'art' of statistics, should be taken into account before reaching any kind of sensible conclusion
I think that you - like a few others on this thread - are missing the fact that in those states where SSM was legalized, the reduction in youth suicide rates was significantly greater than in those states where it wasn't legalized. And that occurred right after legalization in 32 individual states over a 10 year period.
So it emphatically was not the weather.