[hnfong]

GP explicitly stated they were referring to what is taught/learned in a discrete maths class.

finite state machine => usually taught in a theory of computation class

strings => wtf?

combinatorics => would be taught in a discrete maths class but I can totally believe that a software engineer has never used combinatorics aside of solving toy problems

trees, graphs => more likely dealt with in a data structures and algorithms course. That said there is a discrete maths slant on how these topics can be approached, but they involve proofs that nobody really cares about when writing software

modulus => those are usually dealt with under number theory classes, besides, unless you're implementing RSA or some cryptographic function, usually the grade 8 level understanding of modulus (i.e. remainder of a division) suffices

[ecshafer]

That area of computation is a subfield of discrete mathematics, and these are all topics I looked up in my old discrete mathematics textbook.