Now the numerous, persistent student mistakes about facts and opinions I have been seeing the past several years make more sense! (Assuming that McBrayer's claims are correct.)
After posting about this on social media, I received some comments which seemed to indicate widespread (and intense!) disagreement about the existence of moral facts. Most moral philosophers believe that there are moral facts, for a variety of reasons. Among other things. it is difficult to even make sense of moral disagreements in the first place without positing the existence of moral facts as the objects of disagreement. Also, many non-philosophers who dispute the existence of moral facts in one rhetorical context speak and act as if there are other moral facts in other contexts, which might imply that their thinking is incoherent on this issue.
However, here are two problems with the Common Core curriculum that are independent of the issue of whether there are moral facts: (1) Even if there are no moral facts, this is a substantive philosophical thesis, and needs to be established through arguments; it cannot be established simply through positing a definition of the word 'fact'. (2) Even if there are no moral facts, the common core definitions of 'fact' and 'opinion' are flawed, because they imply that an opinion cannot be true, and because they do not distinguish between facts and people's beliefs about facts.