I disagree wholeheartedly and name Matrix and XMPP with OMEMO as counterexamples. IMO they both fulfil the triple “security, openness, usability”.
Plus: Security software has to be open to be secure. The basis of this fact is Kerkhoffs’s principle from 1883.
Obscurity (on many levels) is the main reason for insecure software.
Furthermore I hate the definitely wrong assessments, that there is any truth in the notions “security versus usability” and “openness versus security”.