personally, I think it's fud, I mean the code is open source, if there was a back door, the community would have discovered it by now
Discussion
although what if the backdoor was a part of the protocol itself, like what if there was a way to spoof for example- a ring signer, and then and im not claiming this is valid, but trace back through that, for example? i havent looked at the whitepaper so i dont have the topology of the draftspec handy or in my memory :)