Do you trust that the source code for the Signal iOS app on Github is the source code of the application you're running on your phone? All you really have to go on is the word of each company.
On one hand, we have moxie and a team of people with no evidence against their integrity. On the other hand, a culture that responds to criticism with aggression [1] and then responds to criticism of that by deleting their communications [2].
TL; DR It's reasonable to trust Signal's iOS closed-source app while distrusting Facebook Messenger's also closed-source secret mode app.
[1] https://www.nytimes.com/2018/03/30/technology/facebook-leake...
[2] https://fortune.com/2018/03/31/facebook-employees-are-report...