Hybrids are usually infertile, but not always. A lot depends on the distance between the species of the parents.
http://www.messybeast.com/genetics/hybrid-mammals.html From your link:
The age of the skeleton, determined by radiocarbon dating, showed that full Neanderthals had apparently been extinct for at least 4,000 years before the boy was born. "This is no love child," Dr. Trinkaus said, meaning that this was not evidence of a rare mating but a descendant of generations of Neanderthal-Cro-Magnon hybrids.
Instead of seeing a species split into two, we're talking about it splitting into three. Since we seem to have only one sample of a hybrid line, could the child be an example of a child with some kind of growth disorder? Did you see my "Egil's bones" post?
"...could the child be an example of a child with some kind of growth disorder?"
Yes, it certainly could, I am looking forward to hearing the result of tests.
"Did you see my "Egil's bones" post?"
No, I didn't. Link please.