To be clear, that only applies to the julia runtime (e.g. garbage collector, interface to LLVM, etc.) which is written in C/C++ and the packages in the sysimage, but the code generated by the julia compiler at runtime shouldn’t be affected by how the julia executable itself was compiled, as that’s always targeting the current CPU (unless you explicitly choose a different target with julia -C).
Based on the previous section of my reply, if you’re losing some performance it’s mainly in julia’s runtime, not really in the JIT-generated code. Playing with LTO/PGO (which is not currently easily doable in julia anyway) showed some performance gains in terms of compilation time: