It's likely that had they tested poly men that they would have seen the same response for their multiple partners.
In science, that's known as a "hypothesis" and not a "finding." When you do a study on a specific group of people, it's bad science to generalize outside the test group.
Honestly, we have no idea what they would find if they studied the brains of poly men, or men who have a history of cheating. It could very well be that poly men have fewer oxytocin receptors, and so their attraction to other women is less diminished under its influence. It could be that men who habitually cheat have almost no oxytocin receptors, and so they're as attracted to other women as they are to their partner(s).
Actually, I would find THAT study to be fascinating. It would go towards settling the "hard wired or not" question of poly inclination.
The research is valid in confirming that we have, and which compounds mediate, pair bonding in out nature. It affirms that men's brains can be programmed to diminish the attractiveness and appeal of other women under the influence of oxytocin.
I don't think anyone needed a study to confirm "that" we have pair bonding, but this study didn't do that anyway. It
assumed that these men were pair bonded, because the men said "this is my pair bonded partner."
At most, the study confirmed that oxytocin is involved in pair bonding. That's hardly worth a publication, it's been known for decades.
All it showed is your final point, that attractiveness of non-partners diminishes under the influence of oxytocin in men who identify as monogamous. And sure, that's a valid finding, and it may be interesting to the people who are interested in that kind of thing...
but
It says nothing about monogamy.
The hypothalamic peptide oxytocin (OXT) has been implicated in mediating pair bonds in various species, but as yet, we know little about neurobiological factors that might act to promote fidelity, especially in men. Here we provide evidence for a mechanism by which OXT may contribute to romantic bonds in men by enhancing their partner's attractiveness and reward value compared with other women.
And we still know little.
This type of study is the reason why people in the hard sciences have so little respect for psychology and sociology as a "science." If you want to be taken seriously, you have to start with demonstrable assertions. If your whole study is based on a false assumption, then you haven't proven anything.