> 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.
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.
It's worth noting that some (maybe even many) journal open access options do not allow the authors to keep copyright, even if they pay money! Open access ≠ keeping copyright.