So this probably just shows my ignorance, but I was under the impression that modern browsers isolated running js to running on a single processor? The only par

So this probably just shows my ignorance, but I was under the impression that modern browsers isolated running js to running on a single processor? The only part of the article to touch on this was the part talking about the operating system possibly running on another core. It seems like the elephant in the room. Is it just not true any more that browsers isolate js to a single thread?