A third-party audit, like mentioned in a sibling comment, would be great. Many claims are verifiable, however, especially the claims that data remains on your device - just run it without internet, or sniff the traffic.

Sniffing the traffic does not help if the content that leaves your phone is encrytped by applications that are binaries.

why open source it? where else would/could it be used?

Now, I would like a third party to audit it. That would be great.