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I think it is idiomatic.
V-는 것을 모르다 =
to not-know (모르다) anything (것, things) which concerns V-ing (V-는) =
To not-know anything about V-ing =
To have no idea about V-ing =>
To not know how to V *
(* This is not the same as:
"to not have the know-how of/practical skill to" ~ㄹ/을 줄 모르다" also
"to not know a way to "어떻게 + V(으)ㄴ/는지 모르다")