It could actually go either way, based on the title:

Only half of South Koreans willing to marry; even less [than half] want kids

Or

Only half of South Koreans willing to marry; even fewer [South Koreans] want kids

I'm inclined to lean toward the second. "Even less than half" sounds a lot clunkier than "even fewer South Koreans," so it's not surprising other readers assumed the latter.