That's not true, really.
A core console push wouldn't have translated in the NintendoLand presentation at that infamous E3, while leaving The Wonderful 101 completely out of the live show, for example. Baffling to say the least but hey, that's what they did!
And let's not forget half-baked attempts at recovering the Wii audience, not only with the name of the console of course but with stuff like Wii Fit U or Wii Sports Club.
Wii U became a mostly core-targeted platform (and that's already a stretch with the current perception of the term) only later in the platform lifecycle, when it became clear the casual stuff wasn't working like it used to.
But it absolutely was their
attempt at regaining the core audience. They said so quite plainly before it launched. Clearly it failed, but there was a very specific reason why it came with the software it did:
They were in full-on 3DS emergency mode.
Supporting two completely separate platforms has been Nintendo's #1 problem in the post-Wii world, and even in the last few years of the Wii (to work on 3DS launch software). I honestly believe they recognized the need to combine handheld and console divisions and maybe even devices way back in 2009 or 2010, and the Wii U was already at a stage where it would cost them more to cancel it, so they just bit the bullet and put it out while they worked on their longer-term strategy of combining divisions.
But it's very clear to see that both the Wii and the Wii U suffered from the 3DS, and the 3DS launch suffered from the Wii U. As development costs increase throughout the entire industry Nintendo was fighting a losing battle against themselves by splitting up their efforts. They've really said all this before, even as it stems back to the end of the Wii years.
But this is what the NX is attempting to solve. They are focusing on the one problem they can control- their first party output. If they are able to support the NX heavily from launch until the end of its cycle with compelling first party software, then it's likely not going to be a comparable situation to the Wii/3DS/Wii U. Not that it will obviously be the same success as the DS, but there should not be any first party drought which plagued those systems.
That's really the major issue with Nintendo over the past 6 years, and it's exciting that their solution is close at hand (hopefully).