It doesn't bother me one way or another if it is 20nm or 16nm, and I have no technical evidence, however in my gut I want to say it is 16nm. Why else would Nintendo choose this launch timing, and wait to deliver the final dev machines if they weren't waiting on a particular change in tech.
Zelda has been done for a while now, and I am of the opinion that Pokemon Sun and Moon had been back ported to 3DS from the work they did on a Switch game just so they wouldn't miss their anniversary. If Nintendo was waiting to launch the Switch until they got more 3rd party support they would have gotten out the final dev machines sooner.
Are there any alternative theories as to why the Switches design was finalized so late?
Look at the launch dates for the 1st party games. It's pretty obvious Nintendo struggled to finalise enough games in time for Switch's first 6 months.
Plus this is also the official reason provided by Nintendo for releasing in March 2017, that they needed the time to make the software.
The Pokemon theory has very little base in reality considering the timing of the Pokemon releases on DS and 3ds combined with the 3ds launch.