Proxy SSL and ECC ciphers

It's not a function of ProxySSL, but of ANY SSL man-in-the-middle technology. Without getting into the details, ECC is generally deployed with the Diffie-Hellman key agreement protocol, which does not use the server's public and private keys to encrypt anything in the initial handshake. SSL MITM relies on the RSA key exchange, because the server's public key is used to encrypt the third piece of the master secret between the client and server (the other two pieces are relayed in the clear), so anyone with access to the server's private key and a copy of the message can decrypt and access the encryption keys.