Because the ABI is defined and implemented by shared code in the runtime and compiler. There is no need for cross version compatibility between the two backends

Because the ABI is defined and implemented by shared code in the runtime and compiler. There is no need for cross version compatibility between the two backends, only compatibility within the same version. This isn't particularly new either, FWIW. The Glasgow Haskell Compiler also has an unstable ABI that is not standardized, but LLVM compiled code interoperates seamlessly with code generated by the non-LLVM compiler backend (the mechanism by which that is achieved is likely different though.)