I am not sure what they meant to say. In practice, hotels do not normally refuse entry to the lounge for Diamonds and if that lounge has breakfast (most do), that satisfies the breakfast guarantee as well. I am not sure you can demand a breakfast coupon for the downstairs restaurant in addition to the lounge with breakfast unless they already do it as normal practice in that hotel (normal in many international Hiltons).
On the other hand, for Golds, they may be denied entry to the lounge but will be given a breakfast coupon instead so they can keep the zillion Gold "riff raff" out of the lounge which otherwise would get overwhelmed.
I am sure that if the lounge does not have breakfast, then Diamonds will be given breakfast coupons for the restaurant as well. Perhaps others can verify this case.