You don't need to know lambda calculus...you already have functions in C, what's hard about have an unnamed function?
The lambda calculus, iirc, refers to the proof drawn up by Alonzo Church that anything can be computed through pure functions - ie: you can compute anything without mutating state. Do you have to understand that lambda calculus to use functional languages? Hell no. I sure as shit don't understand it, and I can do functional programming. And, I would argue, that few people really understand it, even though tons of people use functional languages. So, you're right, you don't have to know it, but the lambda calculus isn't really referring to the handy lambda function...there's a lot more behind it...that you don't need to understand.