How does Mountain Lion authenticate with Twitter w/o using OAuth? I was kind of confused when I wasn't redirected to the Twitter page, to allow ML using my Twitter account for several things. In this case I had to input my username + password in the pure Mac gui and it just works.

How?! Basic Auth was disabled a few months ago.
Twitter removed basic auth, yes, but it didn't remove Username/Password auth.

See their documentation on xAuth.

In a nutshell, you collect the user's username and password, and on successful verification, store the OAuth token and discard the username and password.

xAuth is explicitly allowed for certain applications, approved only from Twitter. It's not a stretch to say that Twitter would have approved Apple's request to use this in a heartbeat. The lack of ability to access Direct Messages via xAuth is limiting, but ultimately not relevant given the level of Apple's integration.