Not really, no. Depends on the hardware and interface, and approach taken. If the hardware is properly optimized and easy to port TO, like from PC to PS4 or XBox One (or vice versa), porting the code (which is done by some supported standard, like C# or C++, or even Java) is almost as simple as copy and paste, with some minor adjustments, which would take a month to two months tops to sort out and retest.
Like, can we stop living in a fantasy world? I'm not even sure where to begin because practically everything here sounds like wishful thinking. The reason why porting between PC, PS4 and XBone is easy is due to the fact that the PS4 and Xbone is basically running a PC architecture. That does not mean it does not take a lot of time and work to actually get it working and bug free. If you want to have a look at shitty ports you need to look no further than warnerbrothers and their atrocious PC ports, which clearly need several more months of work to get the optimization for the new system right. And even then most PC ports are optimised to be run with a gamepad just like the console version, because it would take even more development time to make proper mouse and keyboard controls that work. Heck, many AAA ports don't even have rebindable keys, which you'd think is one of the easiest and quickest features to implement.
Nintendo generally run their own architecture, so I'd estimate that porting would be quite problematic. GC to wii was easy since the wii was a gimped GC, and from wii to wii U is also easy since the wii u is basically a gimped up wii. What is known about the NX so far seem to suggest that it runs a very different type of use experience, so the might have come that nintendo decided to start fresh, who knows? But I'd make an educated guess that NX will have very different architecture compare to the wii U. Even so, the wii u have a very unique controller so that's one thing they'd have to completely change in the NX port and make sure it works.
If you want to keep dreaming that multiplatform porting is just a quick copy and paste job, feel free to. Sadly, that's not really the reality of the situation, and you only need to look to all the examples within the industry to see that.
There's also concurrent development approach, where the different versions are being worked on simultaneously, and therefore the same logic and algorithms are put in place, but optimized for different hardware. As long as there's enough communication between the teams on the different versions, this also wouldn't take much time.
Which require devoting more people to the project overall. Making proper version handling tools and automating parity between platforms is actually work in the first place and would take time, not to mention this is bloody new hardware and mostly like have it's own quirks that needs to be worked out.
The NX isn't actually out yet, so that's what the larger delay for a multiplatform release comes from. Not the act of porting.
That's an assumption, you don't actually know this. It also makes no sense in the context of any other multiplatform release not on the edge between two systems. This argumentation is also mutually exclusive to:
But, you also can't assume they're trying to release it alongside the NX. For all we know, the game could only be halfway done by the time the NX actually releases, which is why they didn't say such a thing.
And from a business perspective it makes no sense for this delay of the wii u version to the launch of the NX unless they plan on releasing both versions on the same time.