The difference is that cxx is doing codegen to create a C ABI compatible bridge. So the same thing most people already do when they want to interact w/ C++ over FFI.

I believe that TFA is actually saying that the Swift compiler speaks C++ directly without doing any codegen and just handles everything at the compilation level. Same end result but a lot of subtle differences. Not having to do codegen is a big win, especially in Rust where compile times aren't terrific and binary sizes can already bloat up due to the statically linked nature.