It's not a reference to any study. It's common sense, and you see it everywhere, on every scale and throughout history.
What children do you think have a better future on average: Those whose parents love them or those whose parents hate them?
What companies do you think succeed in the long run: Those with people who love working there or those with people who hate working there and want to jump ship?
What countries become the best to live in: Those whose populace dream of moving abroad or those whose populace love their native land?
It's complex. My wife's father-in-law immigrated from Italy to escape the destruction wrought by fascism in WWII and seek economic opportunity. He was part of a diaspora of a small village in Abbruzze that settled around Binghamton, NY. I would say that they all love Italy and they all love the U.S.
Those are people I know very well because I have been to so many parties, dinners, and other events with them. I've seen the same thing with people from India, China, Sri Lanka, etc. I'd assume that it's the normal condition of immigrants.
I guess I'm confused as to who is allegedly providing the counterargument that they should love out-groups more than in-groups?