The source is out his ass, mostly.
Yes, AdMech went to Commorragh seeking for Haemonculus help on fixing the Golden Throne.
Yes, it is hinted that Craftworlds, Dark kin and Harlequins are working together.
Yes, they helped Cawl and other imperial troops escape Abbadon forces through the webway.
But Eldar are a misterious race. They would burn any Imperial world to save a handful of Eldar lives, or in Dark Eldar case just for crap and giggles. Whatever reason they have for helping the Imperium is not altruistic or for some sense of comradery. If they helped them is for ulterior motives yet unknown and the alliance will be over as soon as those ends are met.
- Your daily fix of spanish Dark Eldar tactica, now with 100% more Ynnari and Harlequins.