If they were saying "monogamy is not biologically innate for some people" I would tend to agree. I think a lot more people are biologically poly-wired than they themselves realize, and end up monogamous solely due to a lack of self-awareness and a glut of societal conditioning.
However, monogamy is absolutely biologically innate for some people. I'm closely related to three of them. They've never had the desire for more than one relationship at a time (just sexual or deeper), and they've never had romantic feelings for more than one person at a time. And these are all people who are aware that there are other possibilities and that some people are capable of being in multiple loving relationships without sacrificing anything significant within those relationships (i.e. everyone's needs are met) or are okay with having casual sexual encounters outside of a romantic relationship. They've just never had those feelings or desires themselves. The one who is in his 30's may explore non-monogamy at some point, given the right person/circumstance, but the two in their 60's are unlikely, I think, to suddenly find themselves in a situation where they decide to try some sort of non-monogamy.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack