Presumably it refers to edges in an object graph [0]. EdgeDB seems to be currently deployed as a management, schema, and graph-to-sql translation layer for PostgreSQL.
There is a difference in how large a set `array_agg` would need to process, since `LEFT JOIN` may produce a very sparse relation and NULLs still have to be scanned over.
[0] https://www.edgedb.com/blog/edgedb-a-new-beginning#edgedb.