Good question. Actually, most non-count nouns can be countable under certain circumstances, when they refer to a countable unit of the substance, or a countable variety of the substance.
For example, "coffee" is a non-count noun. When you talk about coffee as a substance, you don't use an article. You can, however, refer to a single cup of coffee as "a coffee." A single cup of coffee is one unit of coffee, and the word "coffee" becomes countable when it refers to individual units.
The word "coffee" can also be countable when it refers to different types of coffee. At a coffee shop, the manager might tell you, "Several of our coffees are imported from Brazil." This means that the shop sells several types of coffee, and some of them come from Brazil. Each type can be regarded as "a" particular "coffee."
The same is true for cereal. Usually, it's a non-count noun, but in this case, the speaker is talking about types of breakfast cereal. The speaker wants to eat something that doesn't resemble any kind of breakfast cereal. Because a lot of popular breakfast cereals are very different from each other, it makes sense to use the plural, rather than just referring to "breakfast cereal" as a broad, general category of food.