I don't think that was an "or" clause, I think it was an "and". The author will retain the copyright, and they will have generous re-use rights (i.e. they will not agree to a restrictive exclusive license).
But Elsevier's lawyers would presumably argue during negotiations that the option to have CC0 distribution of text meets this stipulation, whether in spirit or letter.
I would bet that MIT's choice to divorce from Els came from a combination of ideoligical, budget, and reputational concerns (e.g. the UC system divorced from them as well and MIT now stands in solidarity). So I don't think issue was likely to be decisive.
I think that if the price had been right, all parties would have found a way to argue that the stated terms had been met -- or to simply not address them.
> Copyright: © Cambridge University Press 2018
> This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
So I guess Cambridge is the copyright holder but it's available under CC, which I think qualifies as "generous reuse rights," as originally stipulated.