Enigma wasn't one time pad based (although these were used a lot during the war), enigma does have known weaknesses such as letters never being decrypted to itself. Enigma was based off daily settings (rotor positions, plugboard settings etc), which could be mostly recovered by the end of the war, unlike actual one time pads which are only crackable by bad key hygiene/brute force.
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.
