But I still can't understand why it is prânz for noon. Should it be in translation only as at lunchtime?
The given translation does not make sense. We would say, "they are going into the city at noon" (or, "at lunchtime").
Well, to say 'The go to town at noon' is correct, and means or implies: they go there (to town) frequently or habitually. Whereas 'they are going ....' implies: they're doing to now, soon, today, or once.