I think this is more a matter of scale than anything else. Particularly on NX, there shouldn't be a meaningful difference between developing for a handheld than a console except for the fact that the console will have more detailed assets and a few extra graphical effects enabled. It's not like the whole "HD development" aspect of working on the Wii U, which was as much a matter of moving from a fixed function to fully programmable graphics architecture as it was asset complexity.
In any case, if Nintendo is truly going to move to near-100% cross-device development, it's not a case of "developing it for the handheld and porting up to the console". As is the current situation with pretty much any third party, whether they're working on UE4, Unity or an internal engine like Frostbite, they don't develop for one device and then port, they develop for all devices at the same time and then have a small team responsible for optimising for each platform. Most employees at third party studios could do their job perfectly well without having a damn clue what platforms their game is due to run on, so long as they're given sensible instructions on expected asset complexity, etc. Developing a game that runs across both a Nintendo home console and a Nintendo handheld wouldn't be enormously different. You create assets that are either as detailed as your highest-end target can handle or as detailed as you can afford and then you scale down to the lower targets. You write game logic that can properly run on each device, and then you compile everything together and optimise and go through QA for each one.
