（）。
That's how cunning North Korea (and China too) is.
Literal
That-much North Korea [topic-は] (China [also-も]) cunning [explanatory-なん] is.
The adverb それだけ ('that much,' 'to that extent') points back to something previously described — 'North Korea is cunning to the degree just described.' It's a retrospective intensifier that bundles the previous discourse into a quantity, saying 'this is what that evidence adds up to.' The parenthetical 中国も inserts a second subject with the inclusive も ('China too'), extending the claim without fully restructuring the sentence — a conversational afterthought rendered in written punctuation. The ending なんです is the polite explanatory form, framing the claim as the speaker's explanation or conclusion rather than a flat assertion.