I'm not fully sure what the intent of the joke is, but note that yes, it's true that a header typically just has the prototype. However, tons of more advanced libraries are "header-only". Everything is in a single header originally, in development, or it's a collection of headers (that optionally gets "amalgamated" as a single header). This is sometimes done intentionally to simplify integration of the library ("just copy this files to your repo, or add it as a submodule"), but sometimes it's entirely necessary because the code is just template code that needs to be in a header.

C++ 20 adds modules, and the situation is a bit more involved, but I'm not confident enough of elaborating on this. :) Compile times are much better, but it's something that the build system and the compilers needs to support.

Not a C++ developer, I prefer C. You are right in general however my understanding is that classes which are generic using templates must be fully implemented in header files because of how templates are implemented. That being said this code doesn't appear to use templates so I'm not entirely sure I get it either?

Well, it's even just horrid code, because they're reading user input in some random associated function, so I think, it's safe to say that this is supposed to be horrid code.