I would be curious if you could find a link for that quote, because Rust has always been of the mind that there is no one correct approach for concurrency, and has good support for channels, threads, work-stealing task pools, etc., all intended to serve different use cases. If there is a line like that somewhere in the RFCs, I would expect it to be in the context of "the design of async/await proposed herein is now known to be the right way to do asynchronous I/O in Rust, given Rust's other design choices and constraints [in contrast to the years of other experiments have been tried, including Go-style green threads]". In particular the long term goal is not to make async/await the end-all be-all, but rather to expose a generalized concept of generators which async/await is then built upon.
This is why i'm sad that Rust has baked async/await into the language. Somewhere in the RFCs, it is stated that "async/await is now known to be the right way to do concurrency" or something like that, but to me it is clear that it is at least a decade too soon to say that.