Delta doesn't tell you before you board whether food will actually be available, although they very easily could do so. My travel agency tells me that they can't figure out in advance how much food will be available on a flight. I suspect that they load food based on the weight of the air plane, making the food supply essentially a random function.
Once you are 35,000 feet in the air, it's way too late to buy some food. I recommend that when you fly Delta, you plan on bringing any food you plan to eat: either buy it at the airport or bring it with you. Not only can you not count on their advertised food service, but giving them revenue for an unreliable service only encourages more poor service. Using alternate suppliers is the good, old fashioned capitalist solution. And making something that is both tasty and travel-proof is a fun challenge for someone who likes to cook.