On the main page, it has

All Manager's modules were built to integrate perfectly with Admin theme. Although it is not completely necessary for you to enjoy the main Manager's resources, using them together will get you better results.

which seems to imply that Admin is not required, but the managers_page.info file has it as a dependancy,

dependencies[] = manager
dependencies[] = admin

(Just hoping to get the docs on the main page to match the code's behavior.)

Also not sure that Admin Theme is the correct terminology, I think it's the Admin module in this context.