Essentially it just tries to open the volume with many passwords in quick succession. So there isn't a vulnerability in Truecrypt itself, and complex / long passwords are unlikely to be found with this tool.
A good encryption scheme uses a slow hashing scheme and salt to defeat such attacks.
And for slow hashing schemes that is a fair point, but when you're looking at file decryption, there is nothing to stop parallel attempts happening at once on different machines. Also TC does support using cascading ciphers which can help to slow it down.
Note: I'm not an expert in Cryptography by any means, but this is my current understanding.
The key that is used to decrypt the hard disk is stored inside an area that is encrypted with the passphrase and/or keyfiles. That may use a different algorithm than the actually OTF encryption to the rest of the disk. Thus, you can use a short key with a LONG hash time and protect a LONG key with a short encryption time and voila.
I mean, that's how this service works anyway; you send it the volume header and it attempts to crack it. If it can read the contents of the header, then you can read out the key used to encrypt/decrypt the volume itself.