There's no such thing as 'multi-threaded React'. JavaScript runs in a single thread unless you use web workers, which React doesn't (this was a popular idea a few years ago, but people have since come to accept that it adds overhead and complexity out of all proportion to the problem it's designed solve).

You're probably thinking of Concurrent Mode, which the talk does indeed address. Concurrent Mode is, among other things, a clever way of solving one of the problems introduced by the virtual DOM paradigm: that you have to rerun a lot of user code on every state change that will often block the main thread if you don't break it up into chunks.

More on virtual DOM here: https://svelte.dev/blog/virtual-dom-is-pure-overhead