Well, it's not meant to be an alternative windows shell (as in SHELL=dopus.exe or whatever in system.ini) as it doesn't come with a desktop replacement. Magellan on Amiga replaces workbench entirely.
Hm? DOpus for Windows does exactly that (if you tell it to). Maybe you test drove a system that had that option turned off?
Double-click the desktop and a DOpus lister opens, just like Magellan. And if another program opens a directory window (i.e., the "Find Target" button from file properties), it'll be a DOpus window.