If it's really true that they only needed a "cursor streaming binary file feed with a very limited set of functionality", then why claim in the headline they wrote their own database and begin the article with the observation that doing ACID databases is really hard?

Maybe they really built only what they needed. If so, good on them, but ditch the clickbait title. On the other hand, a lot of people believe that they've built only what they need, only to discover in a week or a month or a year that they underestimated their own requirements. Then they gradually re-learn all the lessons that led to the development of real databases, to their eventual sorrow.