I don't believe Baku forgot about the calamity. When he said he caught a big fish at chaptee 434, it does not mean he caught Lalo's strongest hand, rather it means that he caught a calamity not at later rounds. Baku's 4 J 1 A have the highest probability to overlap with Lalo, because so far High cards hasn't been used and generally high cards are stronger when combined into a hand.
I don't really know, but folding isn't exactly losing because you don't compare cards anyway, so a calamity may not happen because Baku did not 'lose'. Baku's Call sets Baku to lose properly and fulfill the calamity requirement.
Taking a calamity now and learning its nature now is better than leaving its occurence to luck alone in later rounds. Baku really don't like playing a game on luck alone, just like the old maid game.