I don't remember all the details 100%, but the raid wasn't meant to openly battle the Orks in force. It was supposed to be a surgical strike to eliminate either the Waaghboss himself or his most prominent Mek/ Lieutenant, as well as sabotaging the build up of mechanical forces.
So they were prepared for overwhelming numbers of Orks, but more of the Gargants, Stompas, etc than they expected were already operational.
The main problem though, was that they just couldn't find the exact location of their priority targets. After initial sabotage actions the Raven Guard tried to lure out and find their targets, while the Elysians started a diversionary attack. But as the search went on and on without success, the Raven Guard Captain decided to not simply let the Elysians die for nothing and supported them so they could retreat before being destroyed.
Because of that they lost the initiative and allowed the Orks to gather in force, ultimately forcing a complete retreat from the planet and leading to the failure of the operation.
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