I just recevied an DMCA takedown from GitHub because upstream is “unopensourced”

I just received a DMCA takedown request for a GitHub repo of mine because my fork of EdgeFS (I started working on a Illumos / SmartOS port) uses sources of EdgeFS. Apparently Nextenta has (more then 9 months later) has come to the conclusion that they want to "un-opensource" it and want to put the OSS genie back in the bottle.

Is this even possible?

The most interesting excerpts:

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Please provide a detailed description of the original copyrighted work that has allegedly been infringed. If possible, include a URL to where it is posted online.

After DDN’s acquisition of the NexentaEdge source code (as described above), it was renamed “EdgeFS” and was subsequently improperly open sourced under [private] without DDN’s permission.

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Is the work licensed under an open source license? If so, which open source license? Are the allegedly infringing files being used under the open source license, or are they in violation of the license?

No. EdgeFS was improperly contributed as open source without the authorization of DataDirect Networks, Inc. or its wholly-owned subsidiary, Nexenta by DDN, Inc., the copyright owners.

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