It’s actually been available in EE from very nearly the start, and I go back to when EE was in beta. It’s not a trick or a hack. It’s a function of path.php. I remember seeing something in the Docs (or forums) years ago. It’s really more of a server issue in the basic setup. For example, one domain is used to host the EE installation, hence a single installation of EE. Other domains must point to respective directories on the same server, preferably at the same level as the original EE installation. There’s nothing inside those other domain directories except index.php and path.php. The settings in path.php will point each domain to a specific EE template group and templates. Again, it’s a very bare setup but works similarly to how MSM works. MSM goes much farther, though, which has per site capabilities not found in the above example (MSM does some very cool work in handling groups, forums, modules, templates, and so on). Still, it’s all dependent upon the server setup and path.php. It works. It’s legit. It’s messy.