Is this space in some way physically attached to the existing SFD? One or more of existing dwelling exterior walls acting as part of the new kitchen space.
Is this space having a roof covering all or part of the space? Attached to existing structure? Basically becoming an open wall addition.
If the space is using the portion of existing structure as it's own space of the kitchen I would think it's an addition that would trigger AFCI protection.
Also, If you look at 210.8(A)(2)(3) the space outside of main dwelling unit structure is considered within the dwelling unit purview.
Also code does not limit the number of posible kitchens for a single family dwelling. Only that to be a dwelling unit it must have a kitchen. Code also does not specify a difference if a kitchen is in a seperate structure or part of the dwelling unit as to requirements.
So it would appear a kitchen is a kitchen if it has all the components of a kitchen. (If it looks like a duck and walks like a duck and quacks like a duck, it's a duck.)
Only additional caviet is if the kitchen is outdoors ALL receptacle outlets would need to be GFCI protected not just the countertop ones.
I would say if the space is being used as a kitchen for entertaining purposes (as most likely purpose), the same likelyhood of circuit loads exists with the use of multiple plugin appliances that you would find in an indoor kitchen.
The Code, in general, by the required circuit and receptacles, in some way looks to discourage the use of extension cords. And without adequate capacity even in the outdoors kitchen, the use of multiple extension cords could be expected.
So here we are, yet another "what if" that is not fully addressed within the code. So it seems AHJ is left to make the call as to when is a "kitchen" not a kitchen, even if it has all the components of a kitchen.
This is why law libraries are comprised of hundreds of volumes of law books trying to cover all the "what ifs". But I don't believe we really want the NEC to become that voluminous.
Also as electrician we do have some responsibility to protect the public from themselves with regards to the electrical installation and safety, and the courts recognize that if something does happen. (Is it "reasonable to expect" a certain action or activity.)