From the article there is no detail of what problem they were solving, just "we used tech stack A, we ran into issues, so we went with tech stack B". So I have no idea whether MongoDB was a good choice to begin with, or not, or whether the problem needed a relational database or not. Or for that matter GraphQL or AWS Lambda.

I do get the feeling they went YOLO and built their solution on an over-engineered approach with a stack they were unfamiliar with, and when they hit problems, they YOLO'd into another solution without properly evaluating why, which feels like a team of junior developers run amok.

They also seemed to not really understand relational databases, and why foreign keys are kind of important when you are choosing a relation database implementation (yes you can do without them, but it's best to understand them first).