If it’s in Foundation, yes. Swift 6 on Apple OSes now (since a while ago actually) uses the same open-source foundation as Linux. If it’s a proprietary framework (e.g. TabularData), no. It’s simple.
For the rest, almost all Swift packages developed by Apple are fully compatible with Linux, and the documentation of said packages is usually explicit wrt. platform specifics, AFAIK.
But although that's enough for me to want no part of it, that's not what I was gesturing at. When we dig under the "Foundation" to look at the standard library we find that contrary to your assurance what works and how it works varies from one platform to another, just Apple only care about Apple platforms and so as usual they don't think that's worth mentioning.
They do have this information, they just don't publish it on their Apple pages because they're Apple and yet they insist this counts as the Swift documentation - and that should be all the reason you need not to take such "support" seriously.
The Swift standard library would be built on top of that, not the other way around.
Also I’m not sure what the Core thing you’re talking about even is.