Being able to "open a socket" kind of implies you actually get the socket, not just "Oh, if the far end negotiates the protocol we've pre-agreed then you can also send bytes over it". Web sockets "open a socket" in the sense WebRTC does too, but of course that isn't what you want either.

Only Flash actually has an API like BSD sockets where you can do make TCP connection and send an arbitrary protocol over it. Hence, since they want to test protocol compatibility, they used Flash. Doubtless if Flash didn't exist they would provide a handy Python program or something, and 99% of visitors would never run it.