Yeah I need to update that documentation soon. Database authentication is the default and mojoPortal does have an implementation of the ASP.NET Membership Provider. Basically our provider wraps around our business objects instead of talking dorectly to the database like the built in asp.net membership provider and therefore supports all of our supported databases.
Internally though, mojoPortal does use its own business objects like SiteUser directly so mojoPortal could not really work with some other Membership Provider plugged in without some additional work like syncing tables using triggers. We implement their API, but the API is not sufficient in itself to work with mojoPortal as we have our own internal apis as well.
However, in theory it should be possible to use the mojoMembershipProvider in other apps as an integration technique. I can't say that I've ever tried it though, just that it seems theoretically promising.
Hope it helps,