Is NodeJS really Single-Threaded?

NodeJS is evented (2nd line from the website), not single-threaded. It internally handles threading needed to do select/epoll/kqueue handling without the user explicitly having to manage that, but that doesn't mean there is no thread usage within it.

No.

When I/O operations are initiated they are delegated to libuv, which manages the request using its own (multi-threaded, asynchronous) environment. libuv announces the completion of I/O operations, allowing any callbacks waiting on this event to be re-introduced to the main V8 thread for execution.

V8 -> Delegate I/O (libuv) -> Thread pool -> Multi threaded async