Kudos, I guess I was mislead. What you describe makes sense, and then the docstring is correct as is (`set_tmp_ecdh_curve` only affects the curve used for `ECDHE`).

I thought the only certs with embedded elliptic curves would have been ECDSA based ones, and that those curves and keys would only be used for auth, not for any subsequent DH-based session key exchange. Seems not. FWIW, I can't understand why anyone would want to use (EC)DH session key establishment with fixed keys, since it mixes authentication and encryption key material use and thwarts DH in a way .. but well.