From a quick look at the code it looks like something derived from the key is compared with some value on the encrypted disc, i.e. a key verification check. Like you store passwords, you hash the password and compare it with the stored hash to check whether it is valid. You can then, at least theoretically, use the hamming distance as a fitness function and this seems to be what the code does.

  return int(bitset.From(c.qwords()).SymmetricDifferenceCardinality(target_bitset))

What really confuses me is how this can possibly work for a cryptographic function where any change in any one of the input bit is supposed to, on average, flip half of the output bits. But then again I am not curious enough to analyze the code in detail.