Snark Knight makes a good point.
But I think Morris actually touches on something that gets to the heart of it. The Queens (all of them, Summer AND Winter) are all inextricably linked. Consider the Mother's relationship (what one knows, the other knows). Harry even realises this when he goes to Titania to figure out if Mab is actually nemfected. I don't think Titania necessarily has to kill Mab in contested violence exactly. I think there is a deeper, more serious connection. I suspect there is a fail-safe mechanism for the Courts that Summer can utilise in order to stop Mab. Consider the fact that Harry is considered a viable choice to kill Mab because he has Winter. I think the Queens are connected on a deeper level than there respective alignment of Summer and Winter, all to do with what they truly are. The ability of the Wyld Fae to even Choose which side they go to even hints at this i.e. beneath it all they are all linked.
I don't think it is a retcon Morris. Cold Days is pretty recent, as the series goes. As for Mab being pinned at Artic Tor, don't forget she was "healing" Lea. She HAD to be there. Her Knight was there too (a critical chunk of her power) and her most powerful servant was there (Lea). Mab's ability to be in multiple places at once relies on proxies and representatives. Normally she could have attended the border with Summer and Lea at the Outer Gates, or vice-versa (or whatever). Jim says that is why Lea and Mab rarely are in the same place. Mab could have attended the Outer Gates, but she relies on her forces there and the Gatekeeper to hold it. She couldn't risk Lea escaping or not getting better. Summer wasn't attacking but merely waiting at the border, so she wasn't yet required to take the field.
AND she had just been attacked by the Black Council/Circle/Nemesis-infected forces. Not for nothing, she was defending the heart of Winter. Curiously, I wonder why the Mothers almost never intervene? There are a few theories about why they assaulted Arctis Tor at that moment (some involving Time Travel). Some think it was to rescue Lea (a potent asset). Some even think it might have been to with Molly or Slate. And perhaps there were completely different reasons not yet stated.
Also, I think it worth considering that although Mab is the defender of Reality...I am not so sure she doesn't have other plans that are bad for humanity. I don't believe that Summer is just there to hound Mab when her monsters go and eat people. I think they are actively suppressing her, they certainly have been much more the aggressor than Winter has.
Good point G33k. A very excellent reason.