Yes, it does matter. I have little to no interest in Swift if it's walled off into the Apple infrastructure.

I would be very interested if it was opened up and available for Linux.

If there is a standard available then it doesn't matter so much. With the a spec anyone may write their own implementation. Of course, will they becomes the question then, and whether or not people will adopt it on non-Apple platforms.

That is a nice view in theory, but doesn't mean much in reality, even if they release a spec who is going to be able to build an implementation to compare. It will end up like GNUStep. Even languages that are open like Python have trouble maintaining language compatibility across multiple versions. Written specs are also riddled with ambiguities and implicit assumptions which make the creation of a "perfect" clone very hard.