I think the point that the author was making, outside of any criticism to the API itself, is that the hapless programmer may not know that fork() could even ret

I think the point that the author was making, outside of any criticism to the API itself, is that the hapless programmer may not know that fork() could even return -1. The article is pointing out that possibility as documented by the API, not criticizing a failure of the API. Whether or not the API is faulty in this--and I tend to agree with you--is outside of the scope of the article, though only just.