I know you meant that, and you are incorrect. The private key of the SSL certificate is stored on the .onion site, not on the torwallet.net server. Perhaps you should look closer at how socat works: it has the option to serve up its own certificate, but that is not in use here. It is simply concatenating data between ports 443 and 9050, in both directions.
I mean the private key of the SSL certificate of torwallet.net, not the SSL certificate of the .onion site ......
The SSL private key for torwallet.net is stored on the .onion server, and the .onion server does all the encryption and decryption. That's why it's a little slow; all the traffic must pass through tor first to get to and from the remote .onion server
torwallet.net does not have a private key of its own stored on it or in use for it.