Depends what was meant. The thing is we don't really know exactly what happened at Arctis Tor before Harry and Co got there. That creates an issue, a big one. Because we don't actually know the attack got beaten off. maybe they got in and got what they wanted and got out. But if so what did they leave with, (or destroy). Without that info we really don't know enough.
In fact i'm inclined to say that since the Denarian involved got away clean it probably did succeed. Molly could tear a Denarian apart with the Lady's Mantle. What Mab could do if she put her mind to it probably isn't worth thinking about. And thats the problem. There's just no way any of the attackers would have survived if they'd been beaten off by Mab. And Mab was he only Winter survivor not imprisoned or late to the party like the fetches. That suggests to me they got what they where after. But what was it, we all assumed it was Lea they were after but it could well be that lea simply provided the pretext for the attack and was never the real objective, just an obvious excuse either for Mab's benefit or possibly for some of the help along for the ride.
Hell the entire point might have been to turn Mab and the Denarians against each other. I just finished re-reading small favour and the kind of things they could do with Ivy are all short term gains, mass fear and panic and confusion. That's more tessa's style, maybe nasciel suggested it to tessa and she talked Nic into it. Because let's be fair here, whatever the Denarians faults they don't want the outsiders to win, but pitting them against Mab removes any possibility of them helping in the future. Thats a big deal. Stripping Mab of a potential ally would be more than worth giving up Lea from the outsiders PoV.
Then again maybe it was somthing more physical. Maybe Mab has a weapon vault full of scary stuff and they wanted somthing from it. One of my personal theories is that the fae queens could also be the horsemen, (or rather horsewomen), of the apocalypse, presumably if so they have special swords for the occasion.