EDIT: And speak of the devil, Guild's most recent block was orphaned by Discus Fish even though BTC Guild's block was seen by most of the network well before theirs, since they found the following block. Guild's block was seen *14 seconds* before Discus Fish's block by blockchain.info, and 96% of the nodes blockchain.info is connected to reported Guild's block over Discus Fish's.
Selfish mining is alive and well, then. Any indication that DF might have been perfectly well aware that Guild had already reported 298926 before building on their own preceding block (298925)?
Selfish mining is withholding a block until you have built another on top of it already, or waiting for someone else to announce a block before releasing yours. It's unlikely that this was a malicious act, but it could have been. Discus Fish has always had terrible connections to the network due to being in China, which blocks the IPs to a *lot* of pools/bitcoin services which run some of the fastest relaying nodes on the network.
Discus Fish is a PPS only Chinese pool. Selfish mining with ~15% of the network on a PPS pool would mean bankruptcy (assuming they're legit and not running a fractional reserve) due to the significantly higher orphan rates that selfish mining will always cause.
EDIT: Of course that doesn't rule out Discus Fish doing selfish mining, just that it's unlikely to make economic sense. Of all the pools out there, they'd be the easiest ones to pass off selfish mining as standard inefficiency since their stats are limited at best, the majority of the community can't even read their website, and they are in a country with 3rd world connectivity when it comes to worldwide networking.