Zelda BOTW is at 18.01M
The answer is North America market where Zelda BOTW has charted 32 months out of 34 months since it was released on the market.
In some European countries Zelda BOTW is already charting above Pokemon SwSh in weekly basis.
There's no doubt in Japan Pokemon SwSh outsells BOTW greatly but on the ROTW Zelda BOTW is remaining strong while SwSh is already struggling to keep up with the pace.
Pokemon is also very front load seller.
That exactly what happened during SMO vs BOTW in 2018.
SMO had opened at 9M while BOTW switch+WiiU was at 8.2, fast forward SMO is at 16.59M even with official bundled + Mario theme sales promotion while BOTW is at 18.01 without having none of SMO kind of sales deal.
If 3D Mario failed to keep up with BOTW's legs then Pokemon with much worse reputation in that part will performs much worse.
Smash Ultimate, on the other hand, is performing insanely well in NA and doing greatly in Japan.
Zelda seems to be more Popular outside Japan. Where did you get the 18.01 figure from?
Those are official numbers from Nintendo themselves NS version was given in the recent investors report while the last reported sales from Nintendo on the WiiU version was 1.61m
WiiU Version: https://www.resetera.com/threads/legend-of-zelda-breath-of-the-wild-has-sold-over-13-million-units.96763/
Switch Version: https://www.nintendo.co.jp/ir/en/finance/software/index.html
The total brings it to 18m although NS sales were to the end of December so is higher at this point due to the game's continuous sales of at least 1m a quarter since release.