I was curious about this and went back to check. Fall 2015 was actually dry in the area. Not drastically, but modestly BN each month. So I looked at Fall 1997 and 1982 also. Sept 97 was pretty dry and it got wetter as fall progressed with a wet November. 82 was more of a mixed bag with some variation between the airports but you could say average-ish regionally.
For more modest ninos, fall 2018 was a deluge. Fall 2014 slightly dry, fall 2009 mixed month by month, and fall 2002 was wet.
So maybe the nino=wet fall correlation is mostly in our heads from more typical ninos. With the tiny sample size of strong/super ninos, a dry September at least would be in-family.