Code review comment for ~cjwatson/launchpad:simplify-external-translation-messages

A realisation of the query in question (from https://oops.canonical.com/oops/?oopsid=OOPS-6ce2e41d4a1fc3371ecbbfa31fafcc74) is:

WITH msgsets AS
  ( SELECT POTMsgSet.id
   FROM POTMsgSet
   JOIN TranslationTemplateItem ON TranslationTemplateItem.potmsgset = POTMsgSet.id
   JOIN SuggestivePOTemplate ON TranslationTemplateItem.potemplate = SuggestivePOTemplate.potemplate
   WHERE POTMsgSet.msgid_singular = 10087
     AND POTMsgSet.id <> 29306498 )
SELECT TranslationMessage.COMMENT, TranslationMessage.date_created,
                                   TranslationMessage.date_reviewed,
                                   TranslationMessage.id,
                                   TranslationMessage.is_current_ubuntu,
                                   TranslationMessage.is_current_upstream,
                                   TranslationMessage.LANGUAGE, TranslationMessage.msgstr0,
                                                                TranslationMessage.msgstr1,
                                                                TranslationMessage.msgstr2,
                                                                TranslationMessage.msgstr3,
                                                                TranslationMessage.msgstr4,
                                                                TranslationMessage.msgstr5,
                                                                TranslationMessage.origin,
                                                                TranslationMessage.potemplate,
                                                                TranslationMessage.potmsgset,
                                                                TranslationMessage.reviewer,
                                                                TranslationMessage.submitter,
                                                                TranslationMessage.validation_status,
                                                                TranslationMessage.was_obsolete_in_last_import
FROM TranslationMessage
WHERE TranslationMessage.id IN
    ( SELECT DISTINCT ON (COALESCE(msgstr0, -1),
                          COALESCE(msgstr1, -1),
                          COALESCE(msgstr2, -1),
                          COALESCE(msgstr3, -1),
                          COALESCE(msgstr4, -1),
                          COALESCE(msgstr5, -1)) TranslationMessage.id
     FROM TranslationMessage
     JOIN msgsets ON msgsets.id = TranslationMessage.potmsgset
     WHERE (TranslationMessage.LANGUAGE IN (129))
     ORDER BY COALESCE(msgstr0, -1),
              COALESCE(msgstr1, -1),
              COALESCE(msgstr2, -1),
              COALESCE(msgstr3, -1),
              COALESCE(msgstr4, -1),
              COALESCE(msgstr5, -1),
              date_created DESC ) LIMIT 2001;

I'm struggling to see how that fetches the most recent TranslationMessage; as far as I can see the parent query just does a set membership check on a subquery that returns all the associated TranslationMessages with some sorting that doesn't seem as though it can change the end result. What am I missing?