Existing would be the bigger surprise. We've ruled out anything of Earth's mass out to distances of 100 AU and Jupiter's mass to 1000 AU, since those would be visible to telescopic searches and have measurable gravitational effects on known objects that we don't see from observation. It's almost certain that there's nothing that would meet the "clearing the neighborhood" definition of a planet. At most there would be more dwarf planets on the size order of Pluto and Eris and Quaoar, which are common, but those are each comparatively tiny, like 0.2% of Earth's mass.
The article says "78% of places have been searched"... but that's a really weirdly imprecise way of saying that. What are "places" here? Like, the sphere out to 100 AU is 78% of the volume out to 120 AU... but that makes no sense, the possible locations go out to 1000 AU or more.
There are indeed lots of claims of some evidence for another planet existing... but all those claims are different with no consensus, and it's largely fairly specious proposals trying to get some funding.