I’m not a professional academic, but I’m reading that sentence the same way you are.

It sounds like the conclusion here is that not ever being in a relationship by your late twenties AND having a high intelligence predicts childlessness. For that to be interesting/worth mentioning, I would assume not being in a relationship by your late twenties and having mid level intelligence does NOT predict childlessness, or at least not as strongly.

I’m guilty of jumping straight to the comments after not reading the article, however, so there might be more details either supporting/refuting that conclusion that I’m missing, I’m just going off of your quote.

I think that more readable one may be "not having been in a relationship in late 20s is a stronger predictor of subsequent childlessness than scoring high in intelligence."

This seems correct to me, i.e. the current title here is clickbait.

From the paper:

"The results of the present study did not support my hypothesis that intelligence should be negatively associated with childlessness."

In fairness, the somewhat confusing verbiage here is from I assume a Norwegian non-native English speaker.

To their defense, a lot of academic papers even from native English speakers tend to have barely readable convoluted wording here and there. Either for the purpose of making it sound more complicated or to pad the length of the paper, I have no idea. But it is something I have been consciously noticing since a while ago.

Ah, that makes perfect sense to me (both your wording and the conclusion itself), so thank you.