Ok, apparently it wants an elliptic curve key.  It never used to accept anything but DSA and RSA.  So I made an ecdsa keypair and ssh-copy-id to the TB.  Same failure.  I know that X2Go can't handle ed25519 unfortunately, so I don't even have that key for my user.

When setting the Pyhoca session SSH key manually to /home/bill/.ssh/id_rsa or /home/bill/.ssh/id_ecdsa, I get, "Not a valid EC private key file!"

When not setting the Pyhoca session SSH key either way, and password is allowed on the TB server, it comes up with a dialog box asking for password, and when I enter the correct one I get, "Authentication failed!"

Needless to say, all three methods work on the commandline.

Showstopper because I can't configure wifi without NetworkManager and NetworkManager requires a GUI, and this machine is headless.

Researched this to the ends of the Earth, but the issues were X2Go from a Windows client (I'm CentOS), a permissions problem on keys or .ssh, or ssh_config forwarding was disabled.  None apply here.