I think the "pulling it off" here is coercing their megacorp organization into tolerating the idea of open-sourcing something they poured so much time and money into developing. This is also why people have been celebrating Microsoft open sourcing things recently.
"By the end of the year" perhaps suggests that Microsoft got out in front and Apple's hand has been forced into changing their roadmap. I suspect that Apple will struggle with to support cross platform development beyond tossing Swift over the transom. Supporting diverse execution environments is not their core competence historically.
It also suggests that swift version 2.0 isn't out yet and they don't want to deal with open sourcing it until then or have to go through lawyers regardless.
If you followed Chris Lattner on the dev forums he always gave the impression that they wanted it open source from the start but had bigger fish to fry. They reimplemented a ton of the compiler multiple times after finding bugs in the existing language specs. Don't read too far into this that microsoft forced anything. For one, we'll never know and this is at best conjecture.
As for supporting diverse execution environments, I'd argue llvm/clang/webkit proves otherwise. Granted they're not "supporting" it in the sense that they're selling support for it but I'm not sure exactly who would meet your criteria right now.
I think this is where companies like Xamarin and JetBrains can pick up the slack as they have with the Microsoft stack. As long as it's (legitimately) opened-sourced, of course.
