Isn't the definition of idomatic along the lines of "something that is easily understood"?

On the contrary, it refers to something to which the meaning is not immediately obvious to those not aware of the subtext.

Definition of 'idiom': A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements, as in keep tabs on. (...) A specialized vocabulary used by a group of people; jargon: legal idiom.