[jeltz]

No, I do not think this is a good explanation. What you describe is how InnoDB works.

PostgreSQL stores the actual records separately from the primary key index so the actual records won't fragment but the primary key index will. The records themselves will roughly be stored in insertion order. A random primary key in PostgreSQL mainly means that inserts into the primary key index will be more expensive and that it will bloat the primary key index to some extent.