>Does this mean that a web server written in node is running single-threaded?

Yes, which leads to hilarious things like: https://medium.com/walmartlabs/using-electrode-to-improve-re...

"In our tests, however, we found that React’s renderToString() takes quite a while to execute — and since renderToString() is synchronous, the server is blocked while it runs. Every server side render executes renderToString() to build the HTML that the application server will be sending to the browser."

"The average renderToString()call with this configuration took 153.80 ms."