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.
It’s a comparison, what’s wrong with less
“Less” is usually for non-countable objects, while “fewer” is for countable.
“Less slime” vs. “Fewer bees”
The comparison in this case is operating on the word half, not on the subject of the sentence:
You’re definitely right!
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.