Prove it.
That's the problem with closed source code. There's no way to prove there aren't any backdoors.
I understand what you are saying, but you do understand that argument can be used both ways. Why don't you or someone else prove there is a backdoor and take someone's coins remotely and show the community how it works and that Ledgers can be penetrated? You can't, in the same way that I can't prove there isn't one nor am I trying to.