can I use socket.io-client to connect to a standard websocket?

Not sure if this was the case at the time, but socket.io's website now states this directly in the docs.

Although Socket.IO indeed uses WebSocket as a transport when possible, it adds additional metadata to each packet. That is why a WebSocket client will not be able to successfully connect to a Socket.IO server, and a Socket.IO client will not be able to connect to a plain WebSocket server either.

https://socket.io/docs/

Based on our chat, it looks like you were misled by this quote:

The socket.io client is basically a simple HTTP Socket interface implementation. It looks similar to WebSocket while providing additional features and leveraging other transports when WebSocket is not supported by the user's browser.

What this means is that it looks similar to WebSocket from the perspective of client/server code that interacts with the Socket.io client/server. However, the network traffic looks very different from a simple WebSocket - there's an initial handshake in addition to a more robust protocol built on top of WebSocket once that's connected. The handshake is described here and the message protocol here (both are links to the Socket.IO protocol spec).

If you're writing a WebSocket server, you're better off just using the bare WebSocket interface rather than the Socket.io client, unless you intend to implement all of the Socket.io protocol.