It's been pushed by WikiLeaks on Twitter to be used by Ukrainians.
Due to some people believing that WikiLeaks has been hijacked by Russia, they do not trust the app.
So much so, in fact, that I've actually seen the contra-conspiracy asserted: Russia made Assange/Wikileaks push this app precisely to drive Ukranians users away from it and into the arms of something else to which they presumably have more visibility.
And the app is open-source, you can even compile it yourself.