Seems to be about someone creating massive amounts of privkeys and messing with verification (the fact that 51% attacks no longer exist in this approach make that irrelevant, however.) I'm not an expert on cryptocurrencies, but I don't see how that issue could be any more prevalent here than in "standard" (Bitcoin-style) blockchains.
To expand on that: The only way I can imagine a Sybil attack here is if someone created a massive amount of tiny transactions, and the fees required to get peers to validate them would make that approach infeasible.
That's a thing now? :o. Please explain :).