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LLVM does not need to worry about compatibility here because its libc++ is relatively new and was designed with C++11 requirements in mind, whereas GNU libstdc+
by the_why_of_y 12y ago
LLVM does not need to worry about compatibility here because its libc++ is relatively new and was designed with C++11 requirements in mind, whereas GNU libstdc++ is >20 years old and could not possibly have anticipated these new requirements.
OTOH on Linux you're most likely using clang together with libstdc++, and whether that would use old or new ABI would depend on whether the magic macro is defined in libstdc++ or by g++.
The article says "the C++ standard library headers installed by the libstdc++-devel RPM will have a different default value for the _GLIBCXX_USE_CXX11_ABI macro" so that sounds like clang should do the same thing as g++ here.