This rumor was actually confirmed some time ago by The Pokemon Company. The company’s CEO, Tsunekazu Ishihara, confirmed this when speaking to the Wall Street Journal back in September. It was one of the most persistent rumors while Nintendo was trying to maintain silence about what is now officially called the Nintendo Switch.
To be honest, this was the only rumor for a long time that seemed like it had any legs to stand on. The Wii U was practically a large, separated 3DS thanks to its second-screen functionality, so the next logical step would be to actually enable players to take the console with them in some fashion. Of course, most couldn’t predict that it would be so seamless, offering the same experiences whether you were docked or on the go.