Source code
While having the sourcecode doesn't make it impossible for a backdoor to be hidden, most likely in the form of a deeply buried security vulnerability making it more deniable...
Having the sourcecode is nonetheless an improvement over not having it, and therefore relying on binaries instead.
The world is full of compromises...
A car with airbags, abs, roll bars etc may be safer than one without, but its still possible to crash and die...
Having the sourcecode is better than not having it, obviously its not as safe as writing your own code from scratch on hardware you also designed and built yourself, but it's the best option that's practical and affordable.