To be precise, I think the third party is disguised in Shadow because they're Shadowseers, Harelquin psykers. The Chaos forces are attacking each other because Shadowseers are masters at projecting illusions into people's minds to misdirect them into targeting allies, and because they throw hallucinogen grenades around to compound this.
What's happening fits Harlequin tactics precisely, emerging from nowhere to cause anarchy and make the enemy fight themselves.
They would have a sense of purpose in a way demons of the Ebon Dragon wouldn't.
As a result, I think it's likely, if not certain, that Cegorach has bailed us out.
Biel Tan tried to kill Angron, IIRC, so Eldar intervention is not without a precedent.
Unfortunately that bit of Horus Heresy lore is the result of an author who didn't do the research, as Angron was attacked before the Eldar invented the Paths, before they relearned how to use their psyker powers without being eaten by Slaneesh, before they discovered how to use spirit stones, before the Phoenix Lords, before the Avatars of Khaine could be awakened.
That means the Cratworps Eldar had no aspect warriors, warlocks, Farseers, ability to perform divination to find him or precognition to work out how to, aspect warriors; wraith guard, etc, etc
Originally it was unidentified xenos who attacked Angron, who some speculated to be Eldar, but the Horus Heresy author made that piece of fanon ascend even though it's impossible given what else we know about the Eldar
The writers of the Horus Heresy usually forget that the Fall happened simultaneously with the start of the Great Crusade, and that neither the Dark nor Craftworld Eldar existed in anything resembling their 40K selves in 30K; and didn't for centuries or millenia later, depending on what aspect you're talking about.
Which is understandable on some level. The Horus Heresy game and series is about space marines and primarchs. They just couldn't be bothered to do the work to develop completely new bits of world building based on what the Eldar who lived on the Craftworlds and the Webway would logically have been like.
The only Eldar who are similar to their later selves were the Harlequins, who Cegorach protects.