It is not, but it's like 95% of the way there in my experience - most things that are missing are relatively recent additions that have some complex OS interactions, like filesystem I/O with language-level concurrency features.
> Does this change imply better (eventual) support on non-Darwin systems?
Yes. The non-Darwin Foundation version is already rewrite that takes a lot of effort to keep in lock-step with the closed-source Objective-C version, so unifying the implementations in Swift will both reduce the amount of maintenance effort and promote non-Darwin platorms to more of a "first-party" status.
> Also, is Foundation the same thing as what one might normally term a standard library?
It's more of a standard library++, including some things that other languages include in their standard libraries (Date/Time models) but also other things that are common to put in third-party libraries (networking, etc.). You do not need to use Foundation for basic things like arrays or concurrency that are built into the normal standard library.