You're technically correct, but that's not the "C" being discussed in this article. Also, fopen(3) on OpenBSD does document values errno after failure (by refer

You're technically correct, but that's not the "C" being discussed in this article. Also, fopen(3) on OpenBSD does document values errno after failure (by reference to the malloc(3) and open(2) man pages), and I expect that any reasonable system does likewise.