Many recent graph databases are built like this. If you compare ladybugdb vs duckdb, the internals are very similar.
The main innovation is the "REL table". You can think of it as a many-to-many relationship table on steroids with optimizations at the storage layer and join algorithms.
Definition of what makes something a graph database (apart from the query language) is contested. But we seem to be moving towards: build a reasonable relational database and then add a "REL table" to it with join optimizations.