Storage engines are far from elementary. It is one of the most complex and subtle parts of a database implementation, especially if you care about reliability, scalability, or performance. The term "from scratch" usually does not denote outsourcing all of the hard parts of a database implementation to an external library.
I think people were just disappointed that the article was more about writing a database-like API than an actual database.
I think people were just disappointed that the article was more about writing a database-like API than an actual database.