I look at gen markers as whenever there is hardware that is incompatible with the hardware it's replacing. So if all the switch games are playable on the wiiU then it's still Gen 8. If you can't play all the NS games on the wiiU then the NS is nintendos 9th generation console.
in all honestly, the generation naming system is flawed, not cause it isn't good to have generations but because of how the are grouped together. Cause if we really wanna get into it, the PS4 is sonys 4th generation console. The XB1 is MS's 3rd generation console and the NS is nintendos 7th generation console.