They weren't there a few days ago when I checked, so this is a recent development. Thanks to Looneyman for bringing this to my attention!
The stats are exactly what was expected after those screenshots from a while ago, just Carbink's with 50 more to both attack stats. Predictably, the level-up movepool is a copypaste of Carbink's, with Diamond Storm added at level 50. The added effect rate is also confirmed to be 50%, which is pretty decent all things considered. On the TM front, moves that Diancie gets but not Carbink include Psyshock, Hyper Beam and Giga Impact. Woo hoo, get ready to storm ubers with this thing! And in a strange twist of events, Carbink gets Torment and Trick Room but not Diancie. What. It's especially sad considering Carbink actually USES Trick Room.
So, are you disappointed, disappointed or disappointed? I'm not sure whether I'm disappointed or disappointed myself.
Edit: I just realized Diancie learns Trick Room through level-up, but (allegedly) not through TM. So I'm just going to assume Serebii screwed up somewhere and it keeps Torment too.
Edit 2: This presence of Trick Room in the level-up movepool comes at a cost, though: Power Gem. So maybe Serebii messed everything up and it gets Power Gem at level 46, but not Trick Room at all? Not impossible, since the only other Pokémon that gets Trick Room by level-up is PorygonZ, and it makes sense in that case since it fits its dimensional travel flavor.