Fret not. The term “idiomatic” is just MDN’s editorialised and poorly-chosen title for the underlying element, heavily incomplete and simplified past the point of conveying the meaning at all. They should have given up on choosing a single word for it (like they haven’t tried with <s>). The actual spec describes it much more reasonably:
> The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts.
> The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts.