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.

Cereal can be countable and uncountable. The Oxford Learner's Dictionaries include extra information that is useful to learners. https://www.oxfordlearnersdictionaries.com/definition/english/cereal?q=cereal cereal noun BrE /ˈsɪəriəl/ ; NAmE /ˈsɪriəl/ [countable] one of various types of grass that produce grains that can be eaten or are used to make flour or bread. Wheat, barley and rye are all cereals. ​[uncountable] the grain produced by cereal crops

If The item is a type of cereal: So even though cereal is not countable the various types of cereals or are countable. So you can have wheat cereal, rice cereal, bran cereal etc. in this context you can have ‘a cereal’

Like several nouns, cereal can be both countable and uncountable. In the uncountable form, it refers to a quantity of the food, either raw or processed, e.g. some barley, or some porridge. In the countable form, it can refer to a type of cereal such as the wheat, a type of processed food, such as corn flakes, or a brand of cereal. In this case, it could be referring to a type of processed food, or a brand.