The NSA may have developed SELinux... but it's in the mainstream Linux kernel.
This means:
1) It has had review by members outside of the NSA before inclusion into the kernel.
2) The source is available for anyone to view.
3) You can audit the source right now! :)
Because of this, one can reason that there is very low probability for a "NSA backdoor". Having said that, you can audit the source to make sure.
The same can be said for certain implementations of AES such as that found in OpenSSL. I can't vouch for the specific algorithms involved being secure however, so I'll leave that to someone specialized in cryptology :)
SELinux is only as secure as the access policies in place on your system and only certain distributions (like Fedora) actually distribute SELinux policies.