The lighting is incorrect. The shadow makes it look like the post-it note is actually just a ring of paper. A real shadow wouldn't just magically turn white/transparent as it gets further in from the edge, especially since the part that looks odd is tucked under the paper, and, in the physical world, would be darker since it's more hidden from the light source.
You're correct. However, I think the described technique with generated content and transforms of multiple box shadows can be used to achieve realistic simulations of lighting, like the kinds in your blog post.
The author's idea of realistic lighting effects indeed doesn't take into account the perspective of the curl on the note, and therefore looks a bit unnatural.
I’m guessing they used it on a large element. Something like this is definitely meant for a sub-300px element, where the eye would get tricked into thinking there was a page curl.
