Diffie Hellman isn't a public key encryption scheme, it's a key exchange protocol using public key cryptography. These algos are an alternative to RSA or elliptic curve. Why they have separate encryption and signing algorithms I can only guess. They list one of the schemes as a key establishment protocol a.k.a. key exchange, which leads you to believe its optimized for a very small amount of data, perhaps less than a few hundred bytes for establishing a symmetric key which is immune to quantum