Well, no, I wouldn't. Gnosis Prime is claimed by Saim-Hann as an eldar world. We've had this story from at least two different sources now (Codex:Eldar and Planetstrike), and the whole campaign was clearly planned with the assistance of the dark eldar from the beginning. That definitely says "battle brothers" to me.
That's entirely possible, but they still fought "alongside" the forces of Iyanden. The allies matrix describes levels of coordination, not motives.
I've always felt that Blizzard does a really good job of illustrating the thinking that I believe underlies the allies matrix. The underlying question is not, "How much do you like each other?" but, "When a third party threatens both you and someone you hate, how do the two of you respond?" Consider the Mists of Pandaria trailer. The orc and the human do not like each other. They are survivors of a sea battle, shipwrecked on an unknown land, and they immediately begin to hunt each other. When an unknown third party enters the fight against both of them, they stop fighting. That elevates them to Desperate Allies, right there. They proceed to fight against the third party together, though they don't actually coordinate their attacks. That's Allies of Convenience. When that doesn't work and they realize that **** has gotten real, the human hands the orc his makeshift spear and they attack in concert. That's Battle Brothers. But they're still going to go back to fighting each other later.