I highly doubt Splatoon 2 was built from the ground up on the Switch considering the game originated on the Wii U... Like Zelda BOTW, Mario Kart, and Pokken they ported the games over. I believe Mario Odyssey as well was most likely a Wii U title moved over to the Switch.
BOTW and Mario Kart 8 and now Splatoon 2 have shown improvements in performance and resolution over their Wii U counterparts.
I doubt Splatoon 2 started as a port, but it is likely they just ported over the Wii U engine. Probably same for Mario Odyssey- I doubt it was ever considered for release on Wii U but the engine is probably ported from the version on that console.
But I doubt that will really matter, SMO textures and lighting already look far better than they did at the January reveal, and Splatoon 2 got boosted to 1080p 60fps, so clearly Nintendo is optimizing the engines for the new hardware. It may be one of the last steps in development, but it seems to be happening nonetheless.
BotW is likely a case where the game was just rushed to meet the launch date and they couldn't do all of the optimization they would've liked to. Which we saw some evidence of in those performance boosting patches.