Clang, gcc and MSVC are hardly "every major compiler", as there are many others to choose from specially in the embedded systems, real time OSes, classical commercial UNIX and mainframes.
We don't have always the luxury of choosing which compiler to use.
Relying on implementation details of the compiler or provided library is the first trap to writing portable code across OSes and compiler vendors.
Even if it's not required to be a red-black tree, it is de facto a red-black tree on every major compiler.
Moreover, this doesn't really change the parent's point. It must be some sort of ordered associative container, meaning that it's not going to have the performance characteristics of a hash table.