For whatever reason Zelda is not as popular in Japan. While it's the 4th best selling title for Switch globally, in Japan it's more around 16th.
You can see here that it was 13th before the release of Splatoon 3 and Pokemon S/V which both sold much more than it and this was just after the release of Legends Arceus which likely overtook it after this date.
It's possible it breaks this mark, if we're talking about shipped moreso. But I don't think I'd say it's secured unless there's some information on the shipments/digital-sales in Japan specifically that could change my mind.
Physical sales in Japan accounted for 7.3% of BOTW's total sales as of March 2022.
With digital sales as 31 Dec 2022
Zelda BOTW Switch+WiiU = 3.75 million units which is higher than Pokemon Legends, with the discount BOTW received in February it's certainly going to sell more than Pokemon BDSP.