He got me, Lalo bet on the calamity happening means that Lalo choosing 47 was not only because it's his strongest hand, but also because Lalo hoped to see the calamity and also win too (hence his 2 bios move to trap Baku; Lalo wants to win more). Calamity is still within Baku calculation I believe, but Lalo calculating the calamity too and act around that fact isn't within Baku's calculation.
Baku not folding isn't wrong, just as Shion said. Although Baku resolved the law, he didn't resolve the game as a whole which might give the 45 a super small chance of winning when a possible overlap occur. This fact and the fact that It's kinda safe anyway to call (due to Lalo action) change the nature of the call; it's no longer a choice, it's a must (this pisses Baku off).
He ended up looking happy because the game didn't end so easily.