I remember back in graduate school, the university didn't credit my undergraduate course on Discrete Mathematics although I had it at my college. The undergrad discrete math that I took focused more on linear algebra, matrices, and solving system of equations using linear algebra.

In graduate school since the undergrad course was not credited, I had to take their version of the undergraduate course in Discrete Mathematics and in that undergrad course, I learned a lot about graph theory, interesting algorithms such as TSP, the binomial theorem which I constantly forgot from time to time, but one thing that stood out in that course was propositional logic and the accompanying methods of mathematical proofs within it. That version of the undergraduate course laid the foundation for me when it comes to logic and proofs albeit they lack the linear algebra part which was the focus of that version of the course on my college during undergrad.

Later on, that foundation on logic became handy specially during grad courses on computability theory and computational complexity theory wherein math proofs are a normal part of the topics because in those courses, it is not enough that you read and understand the principles and concepts, the theorems, one also has to understand why those theorems are true via the use of mathematical proofs. This logic at play.

Yes, theoretical computer science is not "just theory" in layman's term. The field is actually pure mathematics focusing on the nature of computation and what can and can not be computed , and how hard.