Very awkward English syntax. Using "to do" as an auxiliary in present tense in English often implies habitual action, which is lacking in the Russian, e.g., "Do they go to have lunch very often?" "Yes, they go every Monday."
Why is not 'Are they going for lunch?' a valid translation here? Обедать refers to a specific lunch?
обедать is a verb that means "to have lunch" so it should not be referring to a specific lunch. Looks like there was a mistake in the translation that needs to reported.
I'm not sure whether I'm agreeing or not: "going for lunch" is synonymous with "going to have lunch" - it just depends on how far Duo is willing to extend the idiom.