I know appliance garages that sit on a counter top are included in the GFCI requirement but they don't count toward the require receptacles for the 2' 4' rule.
The 20" rule in 210.52(C)(5) is for the required receptacles to meet the 2 foot 4 foot requirements, it is not a rule for defining whether a receptacle needs to be GFCI protected or not, the general rule is if a receptacle in installed to serve the counter top then is has to be GFCI protected.
If we look at the GP outlet requirement it has the same kind of requirement that a wall receptacle can not be counted if it is over 5?' above the floor, this is because appliances in these areas come with only a 6' cord, so it can't be counted in the 6'/12' rule, on a kitchen counter top it's 20" because kitchen appliances only come with a 24" cord.
Now if the kitchen appliances only have a 24" cord would it reach a receptacle in a cabinet even if the receptacle was only 18" above the counter top? I don't think so as it would have to go around the bottom of the cabinet, so I would say a receptacle put in a cabinet was not installed to serve the countertop, Now look at the appliance garage, it sits in most cases right on the counter top, and a receptacle located in one can't be counted in the 2'/4' rule (210.52(C)(5) but since it does serve the counter top as it was installed for it is required to be GFCI protected because a appliance with a 24" cord sitting on the counter top can reach it.
