Given that NFs being involved in a relationship is positively correlated to overall relationship satisfaction (in aggregate of both partners).
Given that NF is not significantly correlated to neuroticism (or negative emotions, including amount of worry, length of worry, depth of worry - roughly paraphrasing).
Then I conclude with high confidence that NFs do not express behaviors that are destructive in a relationship, including those covered in the OP. That is, the tangible outcome is actually positive.
The only conjunction that I can see for everything that I've read is that the focus is different... and a focus on relationships is a positive trait in relationships. Essentially what Toonia said. This means that the orientation of the neurotic component is simply different. There are a lot of cases in life where this comes into play. You can worry about something, and not have the worry be justified in the present, but if you remove the worry, then the worry would be justified. Equilibriums like that aren't that unusual.
(I'm still questioning the premise. I need to see the questionnaire and full results from the study, including sampling biases, before I'm going to actually build any framework around the claim. Logically, however, the only explanation I have is what Toonia said, plus the potential for equilibrium 'worry', on average.)
(FWIW, I'm agreeing with you too... my Ti just doesn't like the framework of information here... the distinction between "less happy because they worry" and "less happy, but more successful relationships" is pretty huge!)