Disclaimer; I'm a Product Manager at a Backend-as-a-Service company competing with Firebase...
Absolutely was the wrong tool chosen for the job, which the author very willingly admits. That said, this story of trying to scale of startup leveraging Firebase and then experiencing struggles with scalability and extracting data insights/analytics is extremely, extremely common. While Firebase is an amazing product, it's limitations are not widely considered or understood - which makes this story/article really helpful for someone exploring a tech-stack for their next project.
Questions for the author:
1) Has your experience with Firebase left a bad taste in your mouth when it comes to BaaS's in general?
2) Did you move to another managed service?
3) What technologies did you end up adopting to mature your project?
4) Approximately how long and how much did you end up investing to "re-write" your product?