> How is clio using multiple cores?

Using worker threads or spawning multiple processes on Node, and using web workers on the browsers.

> Did you verify this?

Yes :) I wrote Clio, a lot of work is done on optimizing and developing multi-threading/processing

> Nodejs uses single-threaded IO-multiplexing (last I heard), based on events, and JS is inherently single-threaded.

That is no longer the case, on Node there are worker threads, and there are web workers on the browser.

> Also, "speed comparable to C" (or whatever you write): C is compiled to binary. Even highly optimized JS is still interpreted.

That is also no longer the case. JS is JIT compiled, startup time might be slower than C, but if optimized the runtime performance can match that of C (and definitely not always), although it'll use much more resources compared to it.

In this particular example, performance matches C because of tail call optimizations added by the Clio compiler.