The SourceForge page can be a little confusing, but knowing that a DMG file is one of the ways to install software on OS X is just something that you have to learn if you are using OS X.
Having said that, since the Linux AppImage and Windows EXE installation files have the OS mentioned in their names, it might be good if the OS X one did so as well. That would at least make it more obvious. (The same goes for the stable version of Scribus too.)