I don't read it that way...
"One (1) complimentary specialty coffee, soda or alcoholic beverage with a value no greater than $16.00 + gratuity at Seaday Brunch. On ships where brunch is not available, a complimentary beverage will be offered at Breakfast or Lunch in the Main Dining Room. Redeemable on the last day of the cruise. Alcoholic beverages may only be consumed by adults of applicable legal drinking age."
Seems like if they intended it to apply only to breakfast or lunch, they would have said, "On ships where brunch is not available, a complimentary beverage will be offered at Breakfast or Lunch in the Main Dining Room, [comma] redeemable on the last day of the cruise."
In any event, I'm not aware of any ships that don't have brunch at all. I know on longer itineraries with multiple consecutive sea days they will do separate breakfast and lunch instead of brunch on some days. But I don't know what the exact criteria are... maybe on longer itineraries with 3 or more consecutive sea days (like transoceanic repositioning cruises). I know that on closed loop US cruises with just 2 consecutive sea days, I've only seen brunch served on those sea days.