Probably their auditors? Lying about this would be tantamount to (very serious) securities fraud. Not sure what you're basing on your allegations on besides "trust me bro"
Why would lying about having E2EE be securities (as in stock market) fraud? Would that make any lie ever told by a corporation equate to stock market fraud?
E2EE means end-to-end, where the ends are the participants in the chat. They can read it on your phone, but not on their servers. They need their app to separately transmit the plaintext to their servers to read it.
Has anyone actually audited it?