Elsevier open-access fee waived for subscribers?
@Stephan Kolassa has given the technically correct answer.  I will add that all of this is irrelevant to the vast majority of universities, since they don't pay individual journal prices.  They pay for an Elsevier bundle, and Elsevier forces them to keep the amount paid secret.  It seems clear that bundle prices are determined based on what Elsevier believes a university can afford (i.e., will submit to) and are not based on any formula involving numbers of articles.  Thus Elsevier is double-dipping.  The only way they could prove otherwise would be to have a transparent pricing scheme and to allow universities to make public the actual prices paid.

Note that Elsevier has gone far beyond this and even charged for access to articles whose authors paid an OA fee in some cases.