'in the question' is a location, an adverbial of location, not a noun, so this is not an identity.
The answer lies where? The answer lies in the question. 'in the question' an adverbial pointed at 'lies', telling us where the answer is to be found. Ditto your sentence. 'in the question' points at is, the locative sense of 'be', telling us where. Being in a location, you still exist, so in that sense, it is existential.
Unless of course you want to say: The answer (to a previously asked question) is 'in the question' then you establish the identity 'The answer' = 'in the question'. But I don't think you mean that.
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