ACID would have solved this problem though, right? ACID is simpler to reason about. If there is a strong justification for abandoning it, and enough expertise

ACID would have solved this problem though, right? ACID is simpler to reason about. If there is a strong justification for abandoning it, and enough expertise to still make the system work, then fine. But I doubt they had either the justification (I assume their transaction volume could be easily handled with a SQL system) or the expertise. I'm bothered that MongoDB strives to make people feel comfortable abandoning ACID who clearly don't understand what they are doing.