Typedef vs. Using: The Modern Way to Define Type Aliases in C++ In modern C++ (starting from C++11), the using keyword has become the preferred way to define type aliases, replacing the older typedef ...

The IDL typedef keyword allows typedef declarations that are very similar to C-language typedef declarations. /* IDL file typedef syntax */ typedef [[ [ idl-type-attribute-list ] ]] type-specifier ...

I believe your alternative is exactly equivalent, so the reason they didn't do it that way is just that they preferred their way for whatever reason. Presumably they found it familiar and readable and ...