The article should be titled "Why Figma HAD TO reinvent the wheel with PostgresSQL". When you have a legacy system and not enough time, or will to move off of it the only option is to get inventive and build with what you have.

There is always a price. In this case the database team did something quick, cheap and easily. But the Application teams now have to deal with handling all the nuaces of the system. Maybe Figma has more people in these Apps teams with time on their hands to handle it.