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.
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.
[1]: https://asciiwwdc.com/2014/sessions/101 [2]: http://www.drdobbs.com/architecture-and-design/swift-objecti... [3]: https://news.ycombinator.com/item?id=9680982