They've said that before when the DS came out.
Indeed they did. And I don't blame them. And had the DS not taken off like it did, they'd have come out looking like they said the right thing.
What's important regarding the Wii U is that it more or less comprises the bulk of Nintendo's core market, and the console isn't even 4 years old yet, and most owners probably haven't even had one that long. They could dump it in order to focus on NX, but that would potentially entail several risks. Alienating their long-term audience in one fell swoop, at the behest of a new, unproven device, and ditching a piece of hardware which is -despite poor performance- still working for them, would be bad business in any situation.
3DS is entirely different as it's been on the shelf longer, and stretched much further outside of Nintendo's die-hard audience, so any kind of successor will probably be welcomed. In addition, my gut tells me that NX will be closer in terms of marketability to 3DS than to Wii U. But as always I'm open to being wrong.