So, I just looked it up and apparently their official stance is that auditing is questionably effective and thus unnecessary:

Our software is free and open source, while we repute at the moment [it’s] not acceptable to provide external companies with root access to our servers to perform audits which can not anyway guarantee future avoidance of traffic logging or transmission to third parties. On the contrary, we deem very useful anything related to penetration tests. Such tests are frequently performed by independent researchers and bounty hunters and we also have a bounty program.

In other words, their reasoning seems to be:

1. Their software is free and open source, so if it does logs anything, the community would find out, so in this sense the community is the independent auditors;
2. There’s no stopping an audited party from ceasing to log right before the audit and start up again after the audit ends, so an audit is kind of toothless anyway;
3. Regarding penetration tests, they already have independent testing done as well as a bounty program.

Personally, I don’t entirely agree with points #2 and #3 (though I can see their points), but point #1 is fair I suppose. In my opinion, though, it should not be up to the users to hold the company accountable; and there is a difference between penetration tests and log auditing, as the former I believe are merely to check the resilience against outside hacking.

My end impression is that judging from their other documentation and forum posts, the fact that their software is fully open-source, and their past behavior in accordance with their stated values, I think I’m inclined to believe them. However, it is somewhat worrying nevertheless that there isn’t log auditing involved regardless of their actions.

Edit: Clarification

audits are invalid as soon as they finish, there’s absolutely no way to trust any of these companies.