[The_Colonel]

I think you have async mixed up with concurrency / parallelism. Async is all about events as opposed to blocking wait. As an example, Java had predominantly sync programming model, because the thread would be block-waiting for stuff, no matter the fact there can many such threads block-waiting.

[recursive]

I don't think so. Any language that had event handlers would be just as "async" as the OG javascript. First class thread support matters because sometimes it matters which thread the event/continuation runs on.