I agree with you that it doesn't matter whether the attacker generates the "original plaintext" or some "collision plaintext". This is because for passwords, the original plaintext is not stored anywhere, just the hashed MD5 value of the password is stored. That means that the computer can only compare the MD5 value of the entered password with the MD5 value which has been stored. Any text which produces the same MD5 value will be accepted as the correct password. If the original plaintext was stored somewhere, then the attacker would only need to steal the file with the plaintext passwords in it; which is why the plaintext is not stored.