Well, for one thing, the article you are referencing was written in 2008. It also does not mention using the Retention Warehouse for applying retention rules to data archive ADK files. It talks about using the snapshot functionality part of ILM and storing those files in the Retention Warehouse. The snapshot functionality is not data archiving but rather generating a file containing data how it looks like for a particular point in time. That data is not removed from the database.
So, the answer is no, you cannot use the Retention Warehouse to apply ILM rules to ECC ADK archive files.
Best Regards,
Karin Tillotson