That's correct, if you want to keep using CTE, the gurantee can be achieved via something like this

WITH deleted_tasks AS MATERIALIZED (

  SELECT id 
  FROM task_queue
  WHERE queue_group_id = 5
  AND status = 'pending'
  LIMIT 1
  FOR UPDATE SKIP LOCKED

DELETE FROM task_queue WHERE id IN (SELECT id FROM deleted_tasks) RETURNING item_id, id;