They just launched a new handheld last year, there is no way they replace that so soon. They Wii U is dead, they need to sweep it under the rug and this timeline makes good sense, it will be just right to get in on the current gen without taking too much away from PS4 and Xbox One since there are still millions of users sticking to last gen, which many consider Wii U to be anyways.
There is nothing discounting this from being a true dedicated "gaming tablet" either, which might be why 3rd parties were so excited for it, a gaming tablet with Nintendo's expertise and it is scaleable that would excite most companies.
But there is still nothing saying this is a console or a handheld either way. It's all speculation at this point.