One of the benefits of "really functional" code is that without lots of side-effects and mutable state, it is easy to parallelise things. But if "you’re single threaded" as node.js is, you aren't going to get those benefits anyway, s it doesn't matter.

There are performance benefits to the asynchrony that node.js's callbacks give you - i.e. the single thread that you have doesn't block waiting for things to complete and can handle other requests in the meantime. But this isn't really parallelisation.

Javascript does not have threads, so the only way to use it on the server with decent multiuser performance is with async I/O and callbacks. So he's saying people are using NodeJS because it made it possible to write server-side apps, regardless of its other features.