It is also said in the books that Mab would have to forfeit reality in order to take down summer. Directly by Mother Summer, a far more reliable source than Harry.
The Balance between the courts is within the World. The forces on The Outside are nullenvoid for the fact that to use them against Summer would be worst than tantamount to suicide.
Finally Mother Summer also directly states that Titania and therefor Summer Court has their power to protect humanity from Mab.
Look here's the scene.
"I don't... I don't understand" I said finally "White Council intelligence always estimated Mab's troop count at around fifty thousand. There are freaking formations out there with more troops in them than that"
.......
((p332-333))
"The edge of Faerie" she said "Our outer borders. It would have taken you a decade to learn to travel out this far"
"Oh" I said "And... and it's like this"
"In essence" MotherSummer said She stared sadly out over the plain. "Did you think Mab spent all her days sitting in her chair and dealing with her backstabbing courtiers? No Sir Knight. Power has purpose"
"What happens if they get in?" I asked.
Mother Summer's lips thinned. "Everything stops. Everything"
"Holy crap" I muttered. "Does Summer have a place like this too then?"
Mother Summer shook her head. "That was never it's task. Your COuncil's estimate was fairly close counting only those troops protecting the hearts of Winter and Summer. Mab has more than that. She needs them- for this."
I felt like I'd been hit repeatedly in the head with a rubber hammer. "So... Mab's troops outnumber yours by a jillion."
Indeed"
"So she could run you over at any time"
"She could" Mother Summer said "if she were willing to forfeit reality"
I scanned the length of the wall nervously. It looked like it went on forever and there was fighting all along its length.
"You're telling me that this is why Mab has her [pwer? To... to protect the borders?"
"To protect all of you from the Outsiders, mortal"
"Then why does Titania have hers?" I asked.
"To protect all of you from Mab"
I swallowed.
"Titania cannot match Mab's forces but she can drag Mab personally into oblivion with her- and Mab knows it. Titania is the check to her power, the balance"
.....
((p 334))
"Huh" I said.
"Yes?" asked Mother Summer.
"I was sort of expecting... something else."
"Winter and Summer are twi opposing forces of ouw world" she said "But we are of our world. Here that is all that matters. And showing respect to one's elders is never unwise".
......
((p334-335))
Instead a column of carts and litters entered bearing the groaning wounded of the fighting outside, being watched over by several dozen Sidhe dressed in pure white armor marked with bold green and scarlet trim-Sidhe knights of Summer. Medics.
Ok so. Mother Summer is the direct source for several outright statements and confirmations. First purpose. "Power has Purpose", and it isn't Summer's "task" to protect the Outside.
Second Harry's use of the word and term "a jillion" gets the response "Indeed". According to the multiple online dictionaries a jillion means an indefinitely vast number or a fancifully large number. So either Mab has an Infinite number or just an as yest stated number of forces at the Outer Gates. Given Sidhe can't lie and the Mothers are required to be as vague as possible... could be argued either way. But either way regardless it doesn't count towards the Balance of the Courts because of the following statements.
"So she could run you over at any time"
"She could" Mother Summer said "If she were willing to forfeit reality"
That is an outright statement, confirmation, and fact that Mab would have to forfeit reality if she were to use her forces against Summer. There by in every practical and even technical detail, they could not be used in any conflict or measurement of the two opposing forces.
That Mab's purpose is
"To protect all of you from The Outsiders, mortal"
"Then why does Titania have hers?" I asked
"To Protect all of you from Mab
So right there is a statement, confirmation and fact. Thaat Mab's purpose is to defend mortals from The Outer Gates, That Titania's is to protect mortals from Mab. Which is further clarified below.
"Titania cannot match Mab's forces, but she can drag Mab personally into oblivion with her-and Mab knows it. Titania is the check to her power, the balance"
A further outright statement, confirmation and fact that using the explicit term of "The Balance". That Mab would have to forfeit reality, let the Outsiders overrrun the Gates, before Titania would "drag Mab personally into oblivion with her".
They are a
BALANCE.
Finally
"Winter and Summer are opposing forces of our world" She said. "But we are of our world"
Winter and Summer are "opposing forces" of the same world. Neither side would want to forfeit reality to the Outsiders. They are a balance. As states and confirmed by Mother Summer.