I have a feeling this is just looking for a clever way to say “but Linux isn’t as secure as everyone thinks”, which sure, yes. But also, not many people, especially knowledgeable people, are claiming that Linux is “secure”.
And when it comes to “privacy friendly” that depends so much on what flavour of Linux you are using (Ubuntu? a minimal Arch? Tails?) that it’s not really something you can make broad statements about.
Not really. Do you know how many proprietary, company-specific extensions and modules there are of the Linux kernel out there?
Loads of companies choose not to contribute their stuff back upstream. I don’t know why the NSA did originally in the case of SELinux, but I would guess it had to do with transparency, national defense and not carrying the burden of a module / fork solo. They were also not the only contributors even early on, according to the Wikipedia page
Also, if I recall correctly, there was no other option for MAC back them (no AppArmor or Tomoyo).