Is scirra signalling server url only for construct use?

Is the Singling server address of the Sierra site only for construct use?

Or if someone writes a websocket html client program by himself, he can use that signalling server?

The Scirra-hosted signalling server is for Construct use only. It is not for use with other products or services. Obviously we do not want to pay for and maintain a service for something like completely unrelated Unity games.

You can however host your own signalling server, in which case you can do whatever you like with it. But if you want to use it with other products or technologies, you might be better off just writing your own custom server with something like node.js.