If it only made Darwin jails, then it could still run any native software, and would likely end up running a lot of homebrew/macports. (This started by discussing Apple internal servers, so just running their own stuff plus nginx et al. would be fine.) Or, they could take another cue from FreeBSD and illumos and add a Linux personality, or do like illumos and NT and bake in a VM that was seamlessly integrated and use an actual Linux kernel to run stuff.