There's even more confusion. I would almost guarantee that the “I run my 6 sites on WordPress.” commenter was thinking of 6 blogs, and not a single mu install with six mu "sites" each running a single "blog".

In mu a site and a blog are two different things. Each blog is under the control of a specific site. Each Site can control many many blogs. A user can be an admin on a blog, several blogs or a site admin for one or more of the sites in the mu install.

That's gonna help keep the confusion level high. Especially when you consider tossing the word Domain in there which is a completely independent topic.