How gross and disingenuous. If they want to keep their source proprietary to create a moat then just say so. Lying about open source software being shoddy and insecure just totally puts me off.
They are not wrong that it is easier to penetrate open source software, but in regards to a messaging app security is not a big issue due to the most risky stuff being handled by platform provided api and mobile operating systems having a modern security model.