What is the point of this? Any publishing house can just "self certify" that no AI was used. Why would it be necessary to have an outside organization, who can not validate AI use anyway and just has to rely on the publisher.

Writing a book is, in most cases, something which happens between the author and their writing medium, how could any publisher verify anything about AI use, except in the most obvious cases?

The one thing which matters here is honesty and trust and I do not see how an outside organization could help in creating that honesty and maintaining that trust.