What might this example teach us about singular "they"?

I just now encountered the following sentence in a student paper:

Any new parents that have adopted a child of a different race generally embrace their biological parents[’] race.

Having worked with this student already on this specific point of her argument, I happen to know that the biological parents here in question are the biological parents of the adopted child. Of two possible antecedents for the possessive determiner their, child makes the most sense and is the nearer.

Why, then, if singular they is so fully normal in English as is often claimed, do we at first reading tend to assume that the reference here is to the adoptive parents’ own biological parents, and their culture? That some slight double-take is almost always required of a native speaker in interpreting they/them/their as singular has been demonstrated by Anthony J. Sanford and Ruth Filik (“‘They’ as a Gender-Unspecified Singular Pronoun: Eye Tracking Reveals a Processing Cost.” Quarterly Journal of Experimental Psychology 60.2 [2007]: 171–78); but this case, where a genuinely plural antecedent is available, I think goes way beyond the usual mental/ocular hiccup.

If you are an advocate of the singular they, how do you justify this usage? Do advocates of singular they justify it?