If it's already poured and the AHJ didn't hold them up from pouring you're good to go under 250.50 "that are present". Welp, guess it's no longer present now.
They changed wording in about 2008 or 2011 because the way it was worded before that is exactly what was happening. "are present" is what got changed from "if available" or something close to that wording. Before the change - concrete was poured before EC was ever on site and it "wasn't available" and seldom was ever used as an electrode.
Now that it says "that are present", just because it got covered up before you got there doesn't make it "not present". They did throw in an exception for existing construction though, you don't need to tear up a footing just to gain access to the rebar when putting new service into existing construction. New construction you need to plan accordingly though.
The steel that is encased in the concrete would be the rebar mentioned in the code.
250.50 says all the GE at a structure have to be bonded together. however you are dealing with a separate structure here. The new skid CEE is not associated with the existing structure.
I agree sounds like this generator pad possibly is a separate structure and if so it needs it's own grounding electrode system. Question then is whether there is a qualifying CEE or not, and that sounds like it may depend if there truly is a footing or just a slab. If no qualifying CEE then pound in two rods and move on.
A lot of "if's" go along with this for no more then what we know.
You quoted:
"Metallic components shall be encased by at least 50 mm
(2 in.) of concrete and shall be located horizontally within that
portion of a concrete foundation or footing that is in direct
contact with the earth or within vertical foundations or structural
components or members that are in direct contact with
the earth."
I can still very easily see how an inspector can look at a 20" slab of concrete with rebar in it and say it qualifies under a "concrete foundation in direct contact with the earth". Am I missing some civil/structural jargon that precludes a slab from being a foundation?
There is a fairly recent thread where this is under debate. (is a slab a footing, can a slab with thicker perimeter be considered a footing) There is mixed opinions on it, and NEC isn't really all that clear either for some situations.