I think it's also relevant that the xz exploit made use of the fact that by running code before main, they could modify areas of memory that later get turned re

I think it's also relevant that the xz exploit made use of the fact that by running code before main, they could modify areas of memory that later get turned read-only. Any library that does get loaded with dlopen can of course still attack the process it's in, but it has less tools available to it for evading detection.