I'm fully convinced its just like apple's support: they make some vagueish unprovable claims about privacy, and have a functional and shiny app. That's enough for people to overlook all the privacy issues, and build a cult-like fanbase.
Like if anyone walked into a privacy conference and said, "Hey everyone, I'm going to make a private messaging service. I need everyone's phone number!", they'd get laughed out of the room. But because their app looks nice, then people need to develop the cult-like following whenever it gets attacked, because its touching on an unresolved cognitive dissonance of this being a terrible idea.