Did I read the previous chapter wrong, or was there a correction?
I was so sure BK mentioned "5 fingers" when he confirmed the rules, and not "all the fingers".
Anyway, I like how this is reminiscent of that gamble with the peroxide tube. Baku bit his lip to bleed when Kyara got burnt. And now he cuts off his pinky in front of Yakou who broke his hand to become #0.
I think Baku did this just to show his determination to Yakou, as well as his gratitude. Neronero is not worth it, indeed, but it's not really about him. It's about stopping grieving for Kyara, and acknowledging the new #0 as a true ally. The finger landing on the handkerchief is pretty symbolic too.