They didn't want a KDF, as far as I know, but they wanted a hash function with unlimited input size.
Including the username in the hash input gives you guaranteed domain separation between users that you don't get from salts/nonces. Its a generally good idea if you have a hash function with unlimited input size (all modern cryptographic hash functions except bcrypt have unlimited input size).
Including the username in the hash input gives you guaranteed domain separation between users that you don't get from salts/nonces. Its a generally good idea if you have a hash function with unlimited input size (all modern cryptographic hash functions except bcrypt have unlimited input size).