Gee, I can't imagine why all of the rumours abound! :-/
It is believable that Diana was self-centered enough to deliver a bogus spare to the throne - as a payback to the royals - without having any regard for the devastating effects on an innocent child.
Gotta love the image name: jameshewitt_cad.jpg
Notice how Harry's nose slopes inward, while the rest of the family's (including Diana's brother) are more of an outward protrusion.