In the instance of this particular algorithm, the salt must be kept secret, because it is the only unknown in the process from an attackers point of view.
Fair enough, but then the "salt" is not really a "salt" anymore as that term is known from "password salt", because it is no longer a random input value unique to each different password. It is simply a piece of known-plaintext input for every "hashing" session.
I am ignoring the fact that in a general sense an attacker with resources to obtain the salt can also likely log the master password, in which case no attack against the algorithm is necessary.
