Prompt regurgitation. Model was likely instructed to create the app to do this piece "by hand" (or a phrase with similar interpretation) because the author wanted to avoid pulling in a dependency of some kind.
WITH
o AS (
SELECT FROM orders
WHERE orders.id = $1
),
os AS (
SELECT FROM orderStatuses
WHERE orderStatuses.orderId = $1
ORDER BY DESC orderStatuses.createdAt
LIMIT 1
)
INSERT INTO orderStatuses ...
WHERE EXISTS (SELECT 1 FROM os WHERE os.code != $2)
RETURNING ...something including the status differ check...