> Yes but again it's not hot swapping code as in Erlang, the C++ code is unchanged, they just change some xml somewhere.

Right, not on the C++ side, but on the Lua side that WoW uses - you load the new gameplay code that pulls the new data, and override the globals with new functions.

Why does it matter the language? C++ built in the tooling to allow hot swapping, no?

C++ because 99% of the major games are built in that language.