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 BotW is higher than 16th with 3.41mil as of December 2022, it should be 12th I think. It has insane legs in Japan with digital in particular being huge.
Digital should be up to 50% of the total launch sales of TotK. It's preorders haven't just been huge on Comg. On Amazon.co.jp it has been #1 in the charts for months now and has been #1 on the GEO (a huge retail chain in Japan) preorder rankings since preoders opened. Physical day one shipments should absolutely be over 1mil. I think physical week one sales will be at the very least 800k with 1mil being likely.