DH uses two different numbers to derive the secret key. DH is asymmetric because each side has a different key to start with and they end up with the same secret key. I think it's a good way to envision that the symmetric keys they produce as the data.
DH is used to produce the symmetric key for a symmetric algorithm. DH in itself is not a data encryption algorithm necessarily but a way to agree on a secret key via a public network.
You'll the see the same thing with other asymmetric algorithms like SSL and TLS. They use asymmetric algorithms to create a symmetric key.