Yes, it is correct. In this context, 'egg' is uncountable. It refers to the liquid beaten egg used to make Caesar salad dressing. Here, we're talking about a substance (uncountable) and opposed to individual eggs (countable). Remember that most foods can be either countable or uncountable, depending on what form they're in or how they're served. Let's imagine a breakfast buffet - a dish of fried eggs or boiled eggs contains a number of individual items, whereas a dish of scrambled eggs contains uncountable 'stuff'. That's why you'd say 'How many eggs would you like?' if you're talking about the fried eggs, but 'How much egg would you like?' if you're talking about scrambled egg. The same would go for mashed potato as opposed to individual potatoes, for example.

Just as you say, flour is used to make bread. You don't have to say how much flour to use in a general discussion.

Raw egg is used to refer to an ingredient in a recipe when you don't have to be specific about the quantity or the amount.