The thing with closed-source is that the binaries can be backdoored by the vendor on behalf of the NSA. This is not a new practice. With open-source code this venue is less likely to work and needs an additional layer of deception. Sure the NSA may manage to plant vulnerabilities, but flaws will not be persistent in the same way as when they're planted in cooperation with a vendor.

There's currently a push for reproducible builds which further hardens distros against such attacks.