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
JavaScript is single threaded, so is event-model. But Node stack is not single-threaded.
Node utilizes V8 engine for concurrency.