It's plausible to even do a degree in mathematics and never encounter the name 'monoid' - you'll obviously encounter the structure, but my abstract algebra classes just skipped ahead to groups, and then to rings and modules, instead of spending any time on the simpler structures.

Other times, like when studying formal languages, because regular languages are a monoid, the structure is introduced, via noting that you have an empty language and a method to combine any two languages, but not given the name monoid.

So you come across haskell, and it gives a name to things like monoids, monads, functors, and so on, and says that monads and functors come from category theory, and it seems like a valid inference to say that monoids also come from category theory.