>Both Whatsapp and iMessage are proprietary, so it's also the case of "please trust us that we've implemented it the way we claim we did".
This is simply not true, any serious analysis of Signal would be performed on the binaries and not the source code. Having access to the source code does not make it any easier to discover well-hidden backdoors, but it is possible to exploit e.g. compiler behaviour in a way to create a backdoor that is essentially impossible to detect by reviewing source code.
Access to source code might very well make it easier to discover non-intentional bugs, but does not solve the problem of trust.
>Both Whatsapp and iMessage are proprietary, so it's also the case of "please trust us that we've implemented it the way we claim we did".
This is simply not true, any serious analysis of Signal would be performed on the binaries and not the source code. Having access to the source code does not make it any easier to discover well-hidden backdoors, but it is possible to exploit e.g. compiler behaviour in a way to create a backdoor that is essentially impossible to detect by reviewing source code.
Access to source code might very well make it easier to discover non-intentional bugs, but does not solve the problem of trust.