You're making the baseless (and some might say naive) assumption that it is secure to the best of their knowledge. If they really wanted to build trust then they would prove it and leave no doubt in people's minds.
You're making an assumption of guilt. The fact that something isn't open source doesn't inherently make it insecure.
I was thinking the same thing! But you seem to be assuming guilt and I am not. Honestly, I can see it both ways. It just seemed spurious to me to state that if we don't know it's good, it must be bad.