I've considered systemd possibly being a "backdoor" myself. However I can't think of any other linux that's been documented to be trustworthy with top secret information. The links you posted show historical bugs that have been addressed by now.
Consider the environment RHEL gets built in. As the USG is their biggest customer, Redhat has to build the os in an environment that USG considers trustworthy.
As for any vulnerabilities, USG also has rules and regulations to follow. Mainly, compartmentalization of information. Also logging auditing and remote attestation. I'd be more concerned about an inside job. Think about all the people who before Snowden who didn't become famous.
I'm ranting so one more also: Being someone's biggest customer sometimes gives you a say in the direction a product is taking.