So basically the NSA has been granted the same level of access as every low-grade Taiwanese device manufacturer, the Mozilla foundation that wrote the firefox browser I'm using, and probably multiple front companies associated with the PLA. Check.
Still doesn't prove or even suggest there's a backdoor, and as far as I know, even the big-bad NSA would have to send traffic over a network to control my PC remotely. How come nobody has ever seen that traffic? In order for the traffic to be completely invisible, the NSA would by definition also have to have backdoors in Linux that prevent Linux based security monitors from seeing their traffic.
So basically we have two big choices:
1. The NSA has backdoors in everything (Windows and Linux) and the exact same security researchers who find holes in software on a daily basis are too stupid to see what would undoubtedly have to be highly complex rootkit software right in front of their noses. Basically, you think that Bruce Schneier isn't all that bright.
OR:
2. When the NSA wants to do dirty work it uses the exact same exploits that crackers use every day, albeit with probably a greater degree of sophistication since they have a big budget. Since there are security holes in Windows, Linux, OS X, iOS, etc. etc., the NSA can certainly do nasty things, but they don't do it via magic, they do it exactly the same way that everyone else does it.