I mean, it’s still Open Source, right? So it would be pretty hard for them to hide a backdoor or something??

Right but maybe it combined with other tools they have is what helps them with some exploit.
Like they figured out an exploit but needed SELinux as a piece of the puzzle. It's open source
and we can all read the code but we can't see the other pieces of the puzzle.

Come on, put your conspiracy hat on! ;)