Okay, here's my look on things:
If these scientists based this idea on a single look, then they're pretty sore excuses for scientists. However, looking at the head, one could use the cranial pattern to define whether it was human or not. However again, this pattern may be distorted if, in fact, it is a child's head that has been deformed through some incan tradition. This means that unless the scientists know what a deformed child's head looks like, they can't make a basis on a single look, not to mention the number of other things it could be (a primitive ancestor).
However, I don't believe the scientists just took a quick look, I think they would have looked it over, measured distances and such before claiming it wasn't human. Whether they took deformed children or primitive ancestors into account is anyone's guess.
One thing that bugs me is the skull's chemical construction. Obviously it could be some alien molecule but it looks uncannily like what human bone is made out of. But if it was human bone, and some child from ancient inca, shouldn't it be disintegrated by now? But then again, the construction of the face (while not human looking) has all the human features. Whether these features would be shared by intelligent life on other planets is unknown, however we've seen many examples of life on our own planet which doesn't share this construction, so an alien under different atmospheric conditions will likely have a different construction. To point out the non-human nature of it's appearance, a child's head has different proportions and teeth to an adult, giving greater weight to the incan child argument, however I agree it's a little unsettling. Also I believe the picture distorts the actual size of the eyes.
Finally, we've checked the closest planets for the ability to hold life, our closest star is hundreds of light years away. So if it were an alien, it's obtained the ability to go faster than the speed of light or is incredibly old, much older than we humans can live to and I'm pretty sure, unless they also have gravity modules for their ships, their bodies would have gotten used to the weightlessness of space and broken upon landing on Earth or, if their gravity is different to ours, which it likely is, could have broken anyway.
TL;DR: May be alien, most likely not, if not the scientists should lose their science licences for making such silly claims based on debatable evidence (whatever that is it isn't mentioned). The multitude of possibilities produced by lowly Escapist readers can likely be surpassed by proper scientists and I'm sure they can find some way to explain it. DNA might not be possible but carbon dating would be, if it was of carbon construct. Simply looking at the chemical make-up could tell us it's alien so I think there's a few tests still to run.