I think the aim is not to claim that other pieces violate this collection's copyright, but rather to invalidate the frivolous claims that such melody sequences are copyrighted to begin with.

Either the courts decide they are not, in which case the creators will be happy.

Or they are copyrightable and therefore this collection, even if it is in the public domain, is a prior art that invalidates any claim that a newly used melody is really new/copyrightable.