You are correct, but to use a core file with debug info, the binaries don't have to be bit-perfectly identical. All that has to be equal is the offset of each f

You are correct, but to use a core file with debug info, the binaries don't have to be bit-perfectly identical. All that has to be equal is the offset of each function and statement and that is the case in reproducible builds. Debug info is only "tacked on"; you can strip a binary, generate a core dump and then use the original binary with debug information to debug the core dump.