Assuming the f(ciphertext, key) can produce any plain text (of the same length) from a given ciphertext with the right key, and the keys are chosen sufficiently randomly, you are correct.

Classic XOR encryption is like this. You can make a given ciphertext decrypt to anything you want by XORing the ciphertext with the desired plaintext to get the key.

Therefore, just because you've found a way to decode something to plausible-looking text doesn't mean you've found the correct key.