The difference occurs mostly when you start chaining hashes. In that case, a salt is only relevant in the first hash, whereas the keyed hash needs the key at every hash round.
> You use the 'salt' as the key in the keyed hash.
I thought the two schemes were conceptually different, leading to different engineering tradeoffs: With salts, you assume the attacker can gain access to it. With keyed-hashing, you simply have a second piece of equally-secret information, and you hope it doesn't get leaked.
