You can use a curve for both signing and Diffie-Hellman key exchange. It is with the latter that you can construct encryption using ECIES. But Diffie-Hellman key exchange is constructed as an online back-and-forth thing. Alice needs Bob's ephemeral public key to compute the shared secret, and she needs the shared secret to encrypted data to Bob. But in many instances Bob isn't online and Alice is just sending him an encrypted DM, so she cannot get an ephemeral public key from him.

It is not safe to use your long term public keys to compute a shared secret. At least one side should be using an ephemeral keypair, and that is where I think it is ok to use Bob's long-term private key as long as Alice is using an ephemeral keypair.

And no this wasn't about secp256k1 in particular.