It's more like "We designed the type system to take concurrency into account, and then provided various types ourselves in the standard library (like Send/Sync) to be used with them. We also provide a bunch of concurrency abstractions and tools, in the standard library, but due to the guarantees being more in the type system, others can also write their own. Some of these do rely on OS-level capabilities as well.

In general, very little of Rust is technically "in the language", but to the user, something in the standard library vs the language itself often doesn't matter, it's the same experience either way.