Basically, as soon as something is released as free software, you must have the 4 freedoms: use, study, share, improve. This means, that it is not important, who contributed the code and what else they did, in the context of using that code in any of the 4 freedoms ways. It frees the code from the control of the creator in that sense and the creator cannot take it back. Anyone who releases as free software will give up such control. Oracle once having provided something to free software (My guess: They only did it because the GPL forced them to.) does not mean, that we have to like Oracle. These two things are not connected.

I am not sure about the state of Mnesia, however, I wrote, that it is an example. If I had found a Wikipedia list of DDBMS, I would have posted that. However, I am sure knowledgeable people will be able to name a few more.