This is just a simple matter of timing.
Mab learns of Marcone's kidnapping and names Harry as her Emissary, Titania immediately knows of the appointment because they are linked but Harry doesn't. So in the time before Mab tracks down Harry and notifies him of the favor being called, Titania sends the Gruffs on the hunt. And because Jim has to make Harry's life difficult, he has Mab take her sweet time getting to Harry and informing him.
There is
something else going on, though... as I recall, the deal is that Mab is owed 3 services,
but that Harry gets to choose. He can
Nope out from any given service.
Maybe ... hmmm, maybe Mab's choice is valid *until* he
Nope's out?
(as usual, the mortal's ignorance isn't Mab's problem, and Harry is ignorant that Mab named him Emissary)
So she lets him get attacked.
THEN tells him he's her Emissary.
But she doesn't tell him he can still Nope out... and presumably (with Harry no longer her Emissary) that would stop the attacks.
Very fae, very letter-of-the-law deceptive.
I'd have to go back and re-read both the scene where Harry and Mab originally seal the "3 Services" bargain, and the scene where they negotiate this specific "save Marcone" service, to see if such an interpretation *actually* fits the specific agreements.