You're spot on. GP is repeating the common misconception that "Aurora" is a distinct, from-the-ground-up relational database system which can "pretend" to be either MySQL or Postgres by reimplementing their protocols/front-end from scratch. But that isn't the case at all.
In reality, Aurora MySQL is clearly heavily based on the MySQL codebase, but with major changes specifically to InnoDB's persistence and MVCC layers. Aurora MySQL implements a physical replication system using shared storage, undo, and redo logging. The rest is otherwise compatible with MySQL because it is using a lot of MySQL code.
I haven't read up on Aurora Postgres, but I'd bet it's something similar being applied to the Postgres codebase.
It's entirely possible the custom storage components may share some common code between Aurora MySQL and Aurora Postgres, but a custom storage layer alone isn't a "relational database" which "pretends" to be other DBs. (In contrast, it appears that DSQL actually might be that – a custom system with a reimplementation of Postgres wire protocol.)
Not trying to knock Aurora btw, it's really interesting technology and is quite an achievement by Amazon. But the constant confusion about what Aurora even is creates some support burdens in the DB ecosystem.
In reality, Aurora MySQL is clearly heavily based on the MySQL codebase, but with major changes specifically to InnoDB's persistence and MVCC layers. Aurora MySQL implements a physical replication system using shared storage, undo, and redo logging. The rest is otherwise compatible with MySQL because it is using a lot of MySQL code.
I haven't read up on Aurora Postgres, but I'd bet it's something similar being applied to the Postgres codebase.
It's entirely possible the custom storage components may share some common code between Aurora MySQL and Aurora Postgres, but a custom storage layer alone isn't a "relational database" which "pretends" to be other DBs. (In contrast, it appears that DSQL actually might be that – a custom system with a reimplementation of Postgres wire protocol.)
Not trying to knock Aurora btw, it's really interesting technology and is quite an achievement by Amazon. But the constant confusion about what Aurora even is creates some support burdens in the DB ecosystem.