It's tied to the objective-c runtime to the same extent that other dynamic languages e.g. Ruby, are tied to their runtimes. That hasn't stopped them.
I agree that the poor state of GNUStep and indeed the fact that it even tries to be a compatibility layer discourages users.
However objective-c is not tied to GNUStep. And this line of reasoning is irrelevant to swift.
Neither swift nor its standard library are tied to OSX at all. Moreover the applications for which a Linux versions will be useful, i.e. servers, will not be handicapped by the lack of access to Cocoa.