The reason Meteion and the song of oblivion are not mentioned during the Anamnesis Anyder cutscene is pretty simple: that cutscene came out before Endwalker and the revelations about Meteion were known to the playerbase. It'd be frankly, some pretty confusing and bad "maid and butler" dialogue for Venat and her followers to be all "as you are all already aware, Meteion and her song of oblivion continue even after Zodiark shrouds us temporarily her influence...". (Who's Meteion? Why is Venat telling her followers something they all already know? Why is this exposition so bad?)