Not only did they announce that it would be open source immediately, unlike the source code dump they did with WebKit, Apple made the entire git commit history public from the initial commit.
FWIW, WebKit's complete history is available in its Subversion repository, all the way back to the initial commits in 2001 when they were importing KHTML and convincing it to compile without Qt. Granted, there was around two years after Safari's initial release where Apple simply published source code drops, but the full code history has been public (first in CVS, then in SVN) since mid-2005.
Where? Here's the transcript of the initial announcement [1]. No mention of open source that I see.
Dr. Dobb's had an article in 2014 [2] which said "Swift is proprietary and closed: It is entirely controlled by Apple and there is no open source implementation."
EDIT: I found a HN post from 2015, just over a year after Swift was released, titled "Swift will be open source later this year" [3]. Not immediate at all.