Let's assume that the Gab user used a long and complicated password, that happened to hash-collide with "password". How do you know that Gab would have accepted

Let's assume that the Gab user used a long and complicated password, that happened to hash-collide with "password". How do you know that Gab would have accepted "password" as a password? All we know is that it accepted the long and complicated one. This is obviously moot, as a collision is exceedingly unlikely in this case, but still.