I believe that, in which case “that’s not from Trump” isn’t something they can say matter-of-factly; they’re just guessing (and I think poorly, at that). It’s just palm-reading the post and then giving a definitive, with no caveats, “Nope, Trump didn’t write this.”

I linked to the article because trying to definitively say Trump didn’t write something definitionally has to be reserved for fake posts, not real ones that come from his account with no disclaimer.