You need foreign keys and surrogate keys because you broke your relationships

The article argues that foreign keys and surrogate keys arise because normalization splits a cohesive aggregate into multiple tables.

Once decomposed, developers must rebuild identity, referential integrity, joins, and transaction scope that were implicit in an embedded document.

The point is not that relationships require an RDBMS, but that normalized design forces these mechanisms to restore consistency.

It uses an Order aggregate and third normal form to show why relational models make relationships explicit.