IP isn't the only variable in a decision whether to make something open source. It's seemingly a fundamental building black of Apple's plans for their platforms going forward so I can understand them wanting to keep control of it as long as possible. It's also likely far from "finished", so even if they are planning on releasing it eventually, it probably wouldn't be for another year or two.
I hope it will be open at some point, but it wouldn't surprise me if it never is. At least not in it's current form. However most/all the features of Swift it's using should available soon, so there's nothing to say someone could make a source-compatable version for linux/windows/web at some point.
Also, hopeful there will soon be DSLs for other languages like TypeScript. Might make your job alot easier