Your confusion is pretty reasonable. Users@ and Dev@ used to be mailing lists administered using mailman; they’re now just Google Groups, which reduces the effort required to manage them. The `announce` list is still a mailman list.
I’ve just made a pull request for the subscriptions page that might address some of your concerns; specifically, the text
“Subscription: enter your email here to subscribe to a mailing list”
Now could simply say
“Subscribe to a mailing list”,
and the links to the google groups have text that indicates that this is how you join the corresponding group.
And yes… the gmane graph is broken, you’re right.
John