From a reader:

Dunno about Derb, but I think it’s statistically noteworthy.
My own subjective but (insofar as I know) unbiased inspection would put the equal-distribution percentage at somewhere around a third. And as the missus pointed out, at least in the 1800’s families were routinely larger than merely three kids. That would drag my gut percentage even lower.
But say that in this timespan, 33% of all males are firstborn or only children. The report you cite says “nearly half” so put it around 47%. That makes a firstborn/only male child 14% more likely than not to be president. Not huge news but I’d elevate it to “of interest.”
I eagerly await the “real” opinion from Derb. But there’s mine.