A console generation is defined by an external standard. However NX is releasing due to awful business from Wii U, so its not a product that is demanded by consumers or industry, but born out of sheer necessity.
Looking at wiki and the 3rd gen, Sega released SG-1000 in '83 and the master system in '85, so its not uncommon
for a company to release multiple consoles within the game gen.
https://en.wikipedia.org/wiki/Third_generation_of_video_game_consoles
Also looking at 4th gen, you can throw out the ".5" also. You have turbo grafix 16 in 1987, and Neo geo in 1991, still considered the same gen even though Neo geo was more powerful (like ps4neo/nx will be)
https://en.wikipedia.org/wiki/Fourth_generation_of_video_game_consoles
NX is apart of the 8th gen, as is PS4Neo. But if you don't believe in console gens as we understand it, then clearly Nintendo is in their own lane and only for them can it be considered as "a new generation".
