Yes that’s exactly where I was going with my previous thought @rubydoob
And yes, each member has a single login for all forums and can access all of them.
There are several other much larger forum communities that do something similar to this, the www.bimmerpost.com forums come to mind. Individual forums appear as individual sites with different themes / skins / etc, but at the bottom there are sections like off-topic that are “universal” and appear on all forums allowing cross forum collaboration.
Obviously there are benefits to this from the standpoint of someone who runs multiple forums and for the communities at large as well.