Sybil proofness does not require a CA.
The abstract of your link only states that CAs can be a solution.
An alternative approach would be to have the system function as long as one member remains non-sybil.
> This paper shows that, without a logically centralized authority, Sybil attacks are always possible except under extreme and unrealistic assumptions of resource parity and coordination among entities.
And the proof is a few pages later in a few Lemmas. Note that this paper is where Sybil attacks actually come from, and you can see almost all DHT security papers (eg. Castro’s secure routing paper [1]) assume a CA.
[1] section 3.2 of https://www.cs.rice.edu/~dwallach/pub/osdi2002.pdf