I'm afraid I don't know the history of it well enough to give a good answer, but IIUC what the EFF is claiming here is that CDMs are (potentially proprietary) blackboxes and that EME treats them as such. Therefore, publishers are able to decide which browsers are even allowed to use the CDM which they designate.

As far as CDMs being blackboxes in the standard - you can verify that yourself: https://w3c.github.io/encrypted-media/

It would be good to have an EFF expert explain some of the nuance here though.