Yeah the 20+ million number I'm not defending and don't really have an opinion on personally. Steamspy I think actually usually downplays how popular games are because IIRC the way they calculate is by taking user data and Steam has cracked down on that by making users private by default.
That being said there's definitely reasons to doubt the 20-50 million number, in this steam page
SteamSpy's estimation was 10-20 million back around launch when we know the game would have only been in the 2.6-5.2 million range. Funnily enough, that thread was made one day before the Belgium embassy post.
That being said, I don't think it's much of a story either way. Both games are incredibly popular, and they both have different means of selling. Baldur's Gate 3 has very long legs and there's no reason to think it won't continue to be that way, it is astonishgly popular and will be during the holidays. Zelda on the other hand had a lot of hype before release and funneled a lot of sales at launch. Two equally valid ways to sell your games. I'll also add that the Playstation version of BG3 is $70, so it's possible even people who prefer to play on PS and/or have a bad PC decided to try it on PC anyways.