The NSA, the original primary developer of SELinux, released the first version to the open source development community under the GNU GPL on December 22, 2000.[6] The software was merged into the mainline Linux kernel 2.6.0-test3, released on 8 August 2003. Other significant contributors include Red Hat, Network Associates, Secure Computing Corporation, Tresys Technology, and Trusted Computer Solutions.
It happens, though.
Though this implies that the Department of Defense doesn’t want to use compromised tools, since DARPA is DoD. NSA is also DoD.
People don’t understand that the way a backdoor is usually implemented is not going to be obviously saying “backdoor_here”, neither it will look like a some magic code loading a large string and unzipping it on the fly – that’s sus af. What you will see is some “play video” functionality that has a very subtle buffer overflow bug that’s also not trivially triggerable.
This already happened with WhatsApp.
Honestly I think a lot of backdoors are just unintentionally exploits that are not disclosed