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.
The comparison in this case is operating on the word half, not on the subject of the sentence:
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.
You’re definitely right!