Well, I think its an interestingly worded question; it's not that multiculturalism should "work", like its part of some plan or agenda, I think the argument is that it should simply be a reality. I'm not saying all cultures need to mix and blend and people of all ethnicities and cultures should be living everywhere by mandate or anything
It's quite demonstrable however that tribalism, nativism, and the general 'othering' of cultures that are different from your own can lead to discrimination, oppression, dehumanization, etc via racism bigory and general intolerance. The evidence is all of human history.
I didnt read the entire study itself, but I think you'd need to actually understand the methodology to accurately criticize it... still from what I could make out, I think that just looking at America as a whole is useful on some levels, but not for what he is arguing, especially in a country that is divided into 50 states and more and where the racial and cultural make up can literally vary from state to state or even county to county and sometimes those trends get swept up in the bigger picture of America. This is not to mention the fact that the cultural and racial makeup of America is radically changing in certain directions; for example, white people will not be the statistical majority in the future (probably). I think there are a lot more complex factors that go into explaining this sort of "Turtle" mindset that hes saying is happening where diversity occurs.
It sounds like what the author and the study are really alluding to is what some people would call "forced" multiculturism; diversity for the sake of diversity. I don't think those are actually really significant problems at all, but I mean, America is a white supremacist nation both at its core and in execution; there are way more reasonable answers to explain people being wary of multiculturalism (especially if it seems "forced") than to assume there is a direct correlation between "interracial trust and ethnic homogeneity". He's basically saying "we've always thought more is better, but could it actually be worse?" He's observing a supposed increase in interracial trust within ethnically homogeneous communities (and I honestly question this result entirely, it could be skewed by the methodology of the surveying process) and saying "well, this could mean that making more ethnically
heterogeneous communities would lead to
less interracial trust" when there's no reason to assume that having more heterogeneous (diverse) communities is the
cause; it's a classic case of missing the confounding factor(s), maybe even for the sake of confirmation bias. Really though, that depends on whether the author is using the study to further his own agenda and presents certain things with more certainty than is possible or if the study actually just objectively reports some findings for people to use for w.e they like. I didn't read the study, but it seems likely that its the latter.
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I think its way more simple to say that people who understand each other tend to trust each other and people who are afraid of or misunderstand other people are less likely to trust them, simple as that. There are larger more interesting factors at play that can influence whether or not people of X culture trust and respect and get along with people of Y culture. For example, there's some truth to theorists believing that people who identify as something that has social power (so like, white, male, cisgender) may subconsciously and implicitly see the inclusion of people of other cultures as an attack on their identity, again especially if its perceived as forced. Look at Christian people in America that HONESTLY believe Christianity is under attack. They tend to see America as this Christian, wholesome nation and think that things like Homosexuality, Women's increased liberation, the increase of brown people (of various religions and ethnicities) and more are
literally attacks on their way of life. Because people tend to see parts of their identity as critical parts of their worth as human beings, perceived attacks on classes they fall into become personal attacks. (E.g., race theorists pointing out that institutional racism against black people is factually not only supported by white government, but also
perpetuated and kept in place by regular citizens may result in white people believing that whiteness is under attack)
Also can I just say, this article from 9 years ago is super transparent and was honestly kind of annoying to read.
Broadly speaking people from East & South Asian cultures seem to have no trouble succeeding in the West, but those from African & Middle-Eastern cultures, not so much.