Author seems to be confused on how databases operate (especially since they really should just be using a KV store like Redis), because you're exactly right, the query would be the same as:

    select sites.* from sites join routes using (site_id) where route_id = ?

Assuming sites.site_id and routes.route_id are both primary keys, this query is going to perform identically using either syntax. It'll read 1 row from each table.

They could see a further performance gain by placing an index on routes (route_id, site_id) since the site_id could be retrieved from the b-tree and avoid a table hit entirely. But regardless, query syntax will not affect performance here.