That too, but the fact remains, Mab lied.
Mab didn't lie, she just didn't reveal her whole agenda same as Nic.
Nic bargained for the assistance of the Winter Knight in retrieving an object from Hades vault. He made no promises and asked for none about his plans afterwards because he wanted to be free to kill Harry, whoever Harry brought to watch his back and Anna Valmont in a place where Mab would not be able to intervene. Harry did his part in aiding Nic's plan. Without him, there is no way Anna signs on to Nic's crew and they don't get past the Gate of Ice later. Harry personally gives Nic the item he states was his objective for the job.
Then we get Nic's betrayal and the battles afterwards. Harry, Michael and Anna only survive because Harry made the smart plan to preemptively hire Grey and have him be their ace in the hole. They overcame Nic's hidden agenda, bringing the favor to an end and starting a free-for-all to end the caper. The fight at Michael's was just Nic being a vengeful bad guy.
But Harry(and hence Mab) did keep faith with Nic on his stated bargain. It's just that Nic being treacherous got them so much more.
Mab and Marcone(also Namshiel as we now know) got revenge on Nic for SF.
Mab and Hades got the weapons out of the vault and into capable hands for the future.
Uriel got the preliminary redemption of Nic's squires and a new Knight. He even got Michael more protection by alerting Molly to the holes in his security.
Harry got Bonea out of his head(his bargain with Mab to start the plot) which also let him leave Demonreach and interact with the world again, a fortune in diamonds and the mixed blessing of possessing some Christianity's most powerful holy relics.
By planning for Nic's treachery, Mab was able to accomplish more than balancing her books. And she did it without breaking the letter of their agreed upon bargain.