Given that Americans with and without college degrees are split pretty much 50/50, then we’d expect there to be an equal increase in non-college degrees holder mortality rates if this was caused by changing who got degrees.
I suppose it’s possible that the gender ratio change is the cause of half of the mortality decrease, and the other half is a broad decrease in mortality rates. That would cause it to cancel out in non-college degrees holder mortality holders and double in college degree holders.
Women have had higher life expectancy and lower mortality rate in a number of age brackets, including the one studied in the paper, for much of recorded history, at least in the US. So if more women graduate from college you would expect to see some decrease in average mortality rates (and eventually increase in average life expectancy) among college graduates simply because of that.
Kinda like Bill Gates walking into a bar causes bar patron's average net worth jump up a few million. Funny thing, statistics.
I suppose it’s possible that the gender ratio change is the cause of half of the mortality decrease, and the other half is a broad decrease in mortality rates. That would cause it to cancel out in non-college degrees holder mortality holders and double in college degree holders.