No, you're confused. I knew, I knew, I knew the stupid title of that article would confuse the hell out of people. They didn't find anything, and it wasn't even an amateur astronomer. A professional astronomer produced a model which explained the eccentricity of some Kuiper belt objects by assuming that there was a planetary object ranging from the size of mars to the size of neptune or so orbiting between the Oort cloud and the Kuiper belt. They didn't "discover" anything, nor did they suggest it was hiding behind Neptune. The media's title for the article was sensationalism. In fact, they specifically said that if such a planet exists it could be anywhere in the sky between the Oort cloud and Kuiper belt.