(not saying this is what is/should happen)
It's not quite impossible. They can start with the assumption that all passwords are week, then when you submit your password for the first time, it can do the hash to see if is the right password, + also check the strength of the password at that time (since you just gave them the plain text password).
If it was a strong password they could unlock the account, if is was week, keep it locked and redirect the user to a claim site.
Also, one would think that a strong password would take the hackers longer to crack and thus there would likely only be one claimant for such accounts. I highly doubt anyone was trying to claim my account simultaneous to me (esp. since it was a fairly small amount) yet Mt Gox still sent me through 4 weeks of hoops and tickets and so on. Ridiculous!