modularity of PostgreSQL

Dear all,

I have some questions about PostgreSQL's modularity.
We know that PostgreSQL is  famous  for its  good modularity, which allows users to select which part they need. It also allow developers to easily replace one part without  affecting other parts.  However, as I know, most database engine, like MySQL, has such modularity: layered architecture and small pieces of functionalities. Actually, it's a rule recognized by lots of software design. Why is only PostgreSQL entitled "good modularity"?

Best,
Hope