You can't prove a negative. You can only prove the existence of certain data on a particular server, you can not prove that a company does not have certain data unless you are prepared - and they are willing - to give you full access to audit each and every byte on their systems and to wipe any parts that they can't explain and you can't find a way to decrypt.
Clearly that is not practical so we'll have to take them at their word, as it stands I think that if path is found out to be lying about this that it will come back to haunt them big time.
It probably would help if they had an outside auditor to verify the actions that were taken. Still wouldn't be final proof to anyone who believes that they might still be hiding something but is a step further than just saying "trust us".
